your guide to planets stars and galaxies
TRANSCRIPT
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 114
5 0 0
F A C T S I N S I D
EYour guide to
planets starsand galaxiesby Richard Talcott
A supplement to Astronomy magazine
8129copy 2012 Kalmbach Publishing Co This material may not be reproduced in any formwithout permission from the publisher wwwAstronomycom
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Saturn
Mars
Marsrsquo ruddy appearance arisesbecause the sand on the planetrsquossurface consists largely of ironoxides mdash rust NASAJPLMSSS
Saturnrsquos rings consist of icy particlesranging in size from tiny motes tohouse-sized icebergs NASATHE HUBBLE
HERITAGE TEAM 983080STSCIAURA983081
F U N
Mars boasts the largest vol-canoes in the solar system
although theyrsquore all extinctThe biggest mdash OlympusMons mdash spans nearly 400miles and rises 13 milesabove the surrounding plain
F A C T
Earth may seem extraordinary to those who call it homebut itrsquos not a land of superlatives Earth is neither too hot
nor too cold too big nor too small Itrsquos just right in so
many ways mdash the perfect ldquoGoldilocksrdquo planet Of course
as the only known abode of life in the universe Earth
does have one major claim to being special The other planets in
the solar system leave their marks in different ways
The planets divide into two broad categories terrestrial and
jovian The small rocky terrestrial planets include Mercury
Venus Earth and Mars Mercury the closest to the Sun bakes
at temperatures up to 800deg Fahrenheit at noon But Mercuryrsquos
razor-thin atmosphere canrsquot hold heat at night the temperature
plummets far below freezing Venus most resembles Earth inmass and diameter but a thick atmosphere of carbon dioxide
has led to a runaway greenhouse effect Venusrsquo surface remains
a scorching 865deg F year-round
Earth and Mars are the water worlds of the solar system
Our home planet is the only one with liquid water at the surface
now but spacecraft observations during the past 15 years leave
no doubt that Mars once had loads of surface water Even now
Mars has permafrost and permanent polar caps of water ice
Winds up to 70 mph blow around the ubiquitous martian dust
creating shifting seasonal patterns
The jovian planets mdash Jupiter Saturn Uranus and Neptune
mdash are all gaseous behemoths They consist mostly of hydrogen
and helium the most abundant elements in the universe Jupiter
dwarfs the others It contains more than twice as much matter
as all the other planets combined All the jovian planets pos-
sess ring systems but only Saturnrsquos appears bright Its icy rings
span 170000 miles and measure just 100 feet thick Uranus
and Neptune are the true twin planets of the solar system with
nearly equal diameters masses compositions and rotations
Most scientists no longer consider small distant Pluto to be a
major planet A mixture of ice and rock this world more closely
resembles the thousands of so-called Kuiper Belt objects that
lurk beyond Neptune In 2006 astronomers demoted Pluto to a
ldquodwarf planetrdquo a category that also includes the asteroid Ceres
Planetsof thesolar system
2 Your guide to planets stars and galaxies
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Planet Distance from Sun Orbital period Diameter Mass De (Earth=1) (Earth=1) (Earth=1) (wa
Mercury 039 8797 days 0383 0055 5
Venus 072 22470 days 0949 0815 5
Earth 100 36526 days 1000 1000 5
Mars 152 68698 days 0532 0107 3
Ceres 277 460 years 0075 00002 1
Jupiter 520 1186 years 11209 317832 1
Saturn 958 2946 years 9449 95159 0
Uranus 1920 8401 years 4007 14536 1
Neptune 3005 16479 years 3883 17147 1
Pluto 3948 24768 years 0187 0002 1Note Ceres and Pluto are officially considered to be dwarf planets
Solar system planets
rcurys high density meansre than half of it must be madehe heavy elements iron and
kel NASAJPLUSGS
Venus
Pluto amp Charon
Jupiter
ercury Earth
ranus Neptune
Nearly three-quarters ofEarthrsquos surface is covered withwater Itrsquos what makes our homeworld conducive to life NASA
The orbit of dwarf planet Pluto (left along with its moon Charon) brings it cto the Sun than Neptune for 20 years out of its nearly 250-year-long circuit ESA
Thick clouds blanket Venus so astronomers use radar toits surface The atmospheric pressure there is nearly 100 tithat at Earthrsquos surface NASAJPL
Jupiter is so big that it would take 11 Earths wedged side by side to cross thegiantrsquos girth and more than 1000 Earths to fill its volume NASAJPLUNIVERSITY OF AR
nusrsquo bland cloud tops maskfact that its rotation axis lies in
orbital plane so night and day atpoles last 40 years each NASAJPL
Storms rage in Neptunersquos atmos-phere as they do in the massiveatmospheres of most of the jovianplanets NASAJPL
F U N
Saturn has the lowest densityof any planet In fact if youfilled a solar-system-sized
basin with water the ringedworld would float
F A C T
F U N
Sunlight takes just eight min-utes to reach Earth but more
than four hours to cross thevoid to Neptune and Pluto
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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4 Your guide to planets stars and galaxies
After the Sun and planets therersquos not much else in the
solar system mdash certainly not in terms of mass But
in sheer number (and in a few notable instances
prominence) the small objects hold their own The
biggest of the small bodies actually outrank the small-
est planet Both Ganymede a Jupiter moon and Titan a Saturn
moon have diameters larger than Mercury More than 170
moons have been discovered orbiting the eight planets although
the vast majority are little more than flying boulders
The smaller planets tend to have fewer moons Earth has just
one which formed when an object the size of Mars struck the
proto-Earth ejecting debris that eventually coalesced Mercuryand Venus have no moons and Mars possesses just two small
ones Oddly enough Plutorsquos large moon Charon is half the diam-
eter of the dwarf planet mdash the largest ratio in the solar system
The hefty moons of the gas giants garner most of the atten-
tion Jupiterrsquos four big moons mdash Io Europa Ganymede and
Callisto mdash form a miniature solar system Io ranks as the most
volcanically active object in the solar system Europa hides an
ocean of liquid water mdash perhaps larger than all of Earthrsquos oceans
mdash beneath its frigid ice crust Giant Ganymede also may harbor
an ocean and has a surface covered with intriguing grooved ter-
rain And Callisto sports more craters than any other object in
the solar system At the top of Saturnrsquos family of moons is Titan
which possesses a significant atmosphere and methane lakes
More than half a million asteroids also inhabit the solar system
The biggest Ceres has a diameter of 600 miles Yet most are far
smaller If you add them all up asteroids donrsquot equal the weight
of Earthrsquos Moon Most asteroids circle the Sun between the orbits
of Mars and Jupiter although a few wander into Earthrsquos vicinity
Perhaps the most spectacular small bodies are comets
Billions of these ldquodirty snowballsrdquo lurk in the outer solar sys-
tem If their long looping orbits bring them close to the Sunrsquos
warmth they shed gas and dust The Sun then blows this mate-
rial back to create a long tail Although a cometrsquos nucleus may be
only a mile or two across its tail can stretch millions of miles
Small bodiesof the solarsystem
Titan
Europa
Io
Titanrsquos hazy atmosphere glows as it scatters incom-ing sunlight The atmosphere of Saturnrsquos moon isthicker than Earthrsquos and like ours contains mainlynitrogen NASAJPLSSI
Ridges crack the surface of Jupiterrsquos moonEuropa Such ridges could be sites whereslushy water erupted through the icy surfaceand then froze NASAJPL
More than 100 active volcanoes dot the surfaceof Jupiterrsquos moon Io The plumes can reach 100miles high and spread debris over thou-sands of miles NASAJPL
4 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
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Moon Planet Distance from Orbital period Diameter Den
planet (miles) (days) (miles) (wat
Moon Earth 238900 2732 2160 3
Io Jupiter 262100 177 2263 3Europa Jupiter 417000 355 1940 3
Ganymede Jupiter 665100 715 3271 1
Callisto Jupiter 1169900 1669 2994 1
Enceladus Saturn 147900 137 313 1
Tethys Saturn 183100 189 660 1
Dione Saturn 234500 274 698 1
Rhea Saturn 327500 452 949 1
Titan Saturn 759200 1595 3200 1
Iapetus Saturn 2212600 7933 913 1
Ariel Uranus 118600 252 719 1
Umbriel Uranus 165300 414 727 1
Titania Uranus 271100 871 980 1
Oberon Uranus 362600 1346 946 1
Triton Neptune 220400 588 1681 2Charon Pluto 12200 639 753 1
Major moons
ErosHale-Bopp
Callisto
One of the brightest comets of the past40 years Hale-Bopp wowed observersin 1997 It had a nucleus 25 miles wideand a tail that stretched more than 100million miles BILL AND SALLY FLETCHER
Saturnrsquos icy moon Enceladus reflectsmore than 90 percent of the sunlightthat reaches it the highest percentageof any object in the solar system NASAJPL
PhobosLike most small moons in the solarsystem Marsrsquo Phobos measures just a
few miles across and has an irregularshape NASAJPLMSSS
F U N
Iorsquos active volcanoes andEuroparsquos underground oceanof liquid water both stemfrom enormous tidal forcesmdash which flex and heat themoonsrsquo interiors mdash exerted
by Jupiterrsquos massive gravityF A C T
Multi-ringed impact basins some stretching more than1000 miles formed on Jupiterrsquos Callisto when massiveimpacts left concentric fractures and faults NASAJPL
Potato-shaped asteroid Eros some 20 miles long looks likea lot of other modest-sized asteroids but this object mightone day wander dangerously close to Earth NASAJHUAPL
Triton
nceladus
eyser-like plumes eposited the darktreaks seen on Nep-unersquos moon Triton At aemperature of ndash390deg F Tritonas the coldest surface known in
he solar system NASAJPL
F U N
Saturnrsquos enigmatic outermoon Iapetus has a splitpersonality Half of its surface
appears as dark as freshly laidasphalt while the oppositehemisphere reflects as muchlight as newly fallen snow
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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Protostar
A
ll stars begin their lives in the vast clouds of gas and
dust that litter galaxies like the Milky Way A single
cloud can produce hundreds or even thousands of
stars Something triggers the cloud to start collaps-
ing mdash perhaps strong winds from a massive star ora nearby supernova explosion mdash and gravity works its magic
The cloud fragments and each pocket of material continues to
contract and heat up
The contracting star becomes stable when it starts to gener-
ate energy by nuclear fusion Four hydrogen atoms combine to
form one helium atom Because one helium weighs slightly less
than the four hydrogens combined the reaction creates energy
according to Einsteinrsquos equation E=mc2
The biggest stars contain up to about 120 times as much
material as the Sun They burn hot and use their fuel rapidly
These luminaries may have a surface temperature of 70000deg F
radiate nearly a million times the Sunrsquos light and survive onlya few million years
The Sun shines at about 10000deg F and will last some 10
billion years (itrsquos about halfway through now) The smallest
stars have 8 percent of the Sunrsquos mass and glow at only 3000deg
to 4000deg F mdash so dim that they can shine for a trillion years
Once a star exhausts its hydrogen fuel the end is nigh First
it swells into a red giant expanding to a diameter of hundreds of
millions of miles and cooling to a few thousand degrees It may
tap into more nuclear reactions converting helium to carbon
for example but eventually those
fuels run out as well Stars with up
to about eight times the Sunrsquos mass
eventually puff off their outer layers
and form glowing gas clouds known
as planetary nebulae The star itself
settles down as a white dwarf
More massive stars typically die
in supernova explosions Such explo-
sions scatter the heavy elements built
up during the starrsquos life forming
the raw material for new stars and
perhaps planets The collapsed remnant of the exploded star
becomes either a rapidly spinning neutron star or a black hole
whose gravity is so strong that not even light can escape
Stars inour galaxy
The Sun
Most people think of the Sun as the anchor of our solar system mdash andthatrsquos certainly true It contains 998 percent of all the matter in the solasystem But to astronomers the Sun has even more importance It is theonly star in the universe that appears as more than a point of lightthrough a telescope Detailed observations of the Sun led scientists tounderstand how stars shine how they radiate energy and even howhuge storms wrack their surfaces NASASOHO
The life of a Sun-like star
N49
Heavy elements forged in a massive star spreadout at thousands of milesper second in supernovaremnant N49 One daythese elements may beincluded in a new stellargeneration NASATHE HUBBLE
HERITAGE TEAM 983080STSCIAURA983081F U N
Astronomers divide stars intoseven main spectral classesGenerations of students havelearned the sequence byusing the first letters in thesentence ldquoOh Be A Fine Girl(or Guy) Kiss Merdquo
F A C T
6 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
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Main sequencesolar-type star
Red giant Asymptotic-giant-branch star Protoplanetary nebula Planetary nebula White dw
Spectral Mass Temperature Main sequence Examples class (Sun=1) (Fahrenheit) radius (Sun=1)
O 20ndash120 greater than 55000deg 12ndash25 Zeta (ζ) Puppis
B 4ndash20 17100degndash55000deg 4ndash12 Rigel Spica
A 2ndash4 12300degndash17100deg 15ndash4 Sirius Vega
F 105ndash2 10300degndash12300deg 11ndash15 Canopus Procyon
G 08ndash105 9000degndash10300deg 085ndash11 Sun Capella
K 05ndash08 6700degndash9000deg 06ndash085 Aldebaran Arcturu
M 008ndash05 3100degndash6700deg 01ndash06 Antares Betelgeus
Star characteristics
Stars like the Sun condense out of a gaseous cloud The growing protostar develops a disk (which may form planets)and shoots out material before settling down as a main sequence star converting hydrogen to helium Once the hydro-gen runs out the star swells to a red giant and becomes unstable as an asymptotic-giant-branch star before puffing offits outer layers as a planetary nebula The starrsquos core remains as a dense white dwarf ASTRONOMY ROEN KELLY
Surface of the SunCatrsquos Eye Nebula
Cone Nebula
New stars form from clouds of gas and dust such asthe Cone Nebula Hot stars ionize the surroundinghydrogen gas which glows with a characteristic redcolor NASAESATHE ACS SCIENCE TEAM
When the Sun dies in 5 billion years it may resem-ble the symmetric Catrsquos Eye Nebula Here glowingstrands of ionized gas mark where a dying starrepeatedly shed its outer layers NASAESAHEIC
THE HUBBLE HERI TAGE TEAM 983080STSCIAURA983081
An intricate honeycomb on the Sunrsquos surface marks regionswhere heat (bright areas) rises and cooler material (dark areasinks in a process called convection ROYAL SWEDISH ACADEMY OF SCIE
F U N
To shine as brightly as it doesand nourish life on Earth theSun must convert 600 milliontons of hydrogen into heliumevery second
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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S c u t
u m - C
e n t a
u r u s
A r m
Norma Ar m
C e n t r a l b a r
S a g i t t a r i u s A
r m
P e r s e
u s A
r m
O r i o n
S p u r
Sun
The MilkyWay Galaxy
Head outside on a clear dark summerrsquos night and your
eyes will be greeted by thousands of stars All of them
belong to our galaxy as does virtually everything
else you can see with the naked eye If you let youreyes adjust to the darkness yoursquoll see a gauzy whitish
band running across the sky This is the Milky Way mdash the com-
bined light of countless stars mdash and the feature that lends its
name to our galaxy
The Milky Way is a giant barred spiral galaxy that stretches
about 120000 light-years from end to end but whose disk mea-
sures only some 1000 light-years thick The central bar extends
28000 light-years The Sun lies about halfway between the gal-
axyrsquos center and edge and revolves at approximately 150 miles
per second taking roughly 225 mil-
lion years to complete one circuit of
the galactic hubThe most obvious sights of the
galaxy are stars Astronomers esti-
mate between 200 and 400 billion
populate the Milky Way Galaxy
(most are hidden from view or
extremely faint so a precise count
isnrsquot possible)
Because the hottest brightest stars
are also short-lived mdash and the spiral
arms are the only place in the galaxy
with active star formation mdash the arms stand out The clouds
of gas and dust from which stars form also call the spiral arms
home as do the open star clusters that emerge from them
The nuclear bulge of the galaxy consists mostly of old stars
It measures about 12000 light-years across At the galaxyrsquos heart
lies a supermassive black hole that weighs approximately 4 mil-
lion Suns Surrounding the bulge and disk is a vast spherical
halo that stretches some 300000 light-years
The most prominent members of the halo are globular
clusters These ancient collections of up to a million stars each
were born at the same time as the galaxy some 12 billion to 13
billion years ago They contain few heavy elements because they
formed before supernova explosions had enriched the inter-
stellar medium with them
Structure of the Milky Way
The Pleiades M3
Star formation in Cygnus
F U N
Most naked-eye stars aremassive and highly luminousones that shine across greatdistances But this gives a dis-torted view of the galaxy as awhole In actuality cool dimM-type stars make up abouttwo-thirds of all stars in theMilky Way
F A C T
A stellar nursery in Cygnus harbors many massive young stars Invisibloptical light the DR21 complex shows up when viewed in dust-penetratinfrared radiation NASAJPL983085CALTECHA MARSTON 983080ESTECESA983081
Open star clusters like the Pleiadescontain dozens to hundreds of starsThese groups lie in our galaxyrsquos spi-ral arms and disperse over billionsof years JASON WARE
Globular star clusters have eas long as the Milky Way M3 p500000 stars in a sphere 160 years across S KAFKA AND K HONEYC
INDIANA UNIVERSITYWIYNNOAONSF
The Sun lies in the Orion Spur one of several arms and smaller appenda
where our galaxy creates stars Astronomers name the spiral arms after tconstellation where they appear prominent NASAJPL983085CALTECHR HURT 983080SSC983085CALT8 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
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F U N
Much of the Milky Way Galaxyand its structure remain hid-den to earthbound observersbecause dust chokes the spi-ral arms Itrsquos like being in thewoods and trying to discernthe forestrsquos form
F A C T
F U N
In measuring distances inthe galaxy and the universeastronomers use a unitknown as the l ight-year Itrepresents the distance abeam of light travels in oneyear At 186000 miles persecond light traverses 59trillion miles in a year
F A C T
band of dust cuts through the Milky Way blocking light
om distant stars If not for all the dust the galaxyrsquos centerould shine brighter than the brightest star STEVE THORNTON
F U N
A century ago astronomersthought the Sun occupiedthe center of the galaxy Butcareful studies of globularclusters which orbit theMilky Wayrsquos center and tendto gather in the constellationSagittarius show we livehalfway to the edge
F A C T
F U N
How do astronomers knowa black hole resides at theMilky Wayrsquos center They havefound stars near the centralhub orbiting so fast that theymust be circling an invisibleobject containing 4 millionsolar masses
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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T
he collections of stars gas and dust known as galaxies
form the building blocks of the observable universe
Roughly 125 billion galaxies populate the cosmos and
they come in all shapes and sizes Astronomers divide
galaxies into three major categories spirals ellipticalsand irregulars A spiral has a broad disk containing clouds of gas
and dust and from two to several spiral arms a nuclear bulge of
old stars and a spherical halo that envelops both
Approximately one-third of spirals exhibit central bars mdash a
symmetric concentration of stars and sometimes gas and dust
that crosses the nucleus and connects with the outer spiral arms
(Recent studies show the Milky Way possesses a significant bar)
The diameters of spirals range from roughly 20000 to more
than 100000 light-years and they contain anywhere from sev-
eral billion to several hundred billion stars
Elliptical galaxies appear spherical or flattened in shape
They possess little of the gas and dust seen in the disks of spiralgalaxies so they donrsquot generate any new stars Ellipticals show
the widest range in size of any galaxy type Giant ellipticals can
span 1 million light-years and contain several trillion stars
dwarf ellipticals may be only a few thousand light-years across
and have millions of stars An important intermediate type of
galaxy has a disk like a spiral galaxy but contains no gas or dust
Astronomers call this type of galaxy a lenticular
Irregular galaxies donrsquot show any symmetry or organized
spiral structure The category exists basically as a catchall for
galaxies that donrsquot fit either the spiral or elliptical classification
Irregulars can be big containing up to 100 billion stars or as
small as dwarf ellipticals Astronomers think most irregulars
result from the collisions or mergers of two or more galaxies
The gravitational interactions disrupt normal spiral or elliptical
structure leaving behind a chaotic appearance
Most galaxies belong to groups with dozens of members or
to clusters with up to thousands of members The Milky Way
joins with the slightly larger Andromeda Galaxy to form the
cornerstones of the Local Group a collection of roughly 50 gal-
axies that spans several million light-years The vast majority of
Local Group galaxies are dwarf ellipticals and irregulars Small
groups generally have a few dozen member galaxies but clusters
can contain several thousand galaxies The Virgo cluster located
50 million light-years away is the nearest large cluster to Earth
Galaxies
in theuniverse
The Mice
NGC 4414
Multiple spiral arms wind out from the nucleus ofNGC 4414 Young blue stars throng the armswhile older redder stars populate the nuclearbulge NASATHE HUBBLE HERITAGE TEAM 983080STSCIAURA983081
The ldquoMicerdquo are two spiral galaxies in the process of merging Gravity haspulled material out of each to form long tails while compressed gas cloufuel new star formation NASAESATHE ACS SCIENCE TEAM
M87
A high-speed jet shoots from the heart of the giant
elliptical galaxy M87 (upper left) in the Virgo cluster A black hole of some3 billion solar masses drives this activity NASATHE HUBBLE HERITAGE TEAM 983080STSCIAUR
Giant elliptical galaxies M84 (left) and M86 (right)each contain a trillion stars These two dominate the central regionof the nearby Virgo cluster a collection of some 2000 galaxies NOAOAURA
M84 amp M86
10 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
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NGC 68221760 kly
Ursa Minor Dwarf 225 kly
Draco Dwarf
248 kly MilkyWay
SMC189 kly
NGC 1852020 kly
NGC 1472460 kly
Andromeda Galax
2510 kly
Pinwheel Galaxy (M33)
2770 kly
Leo I818 kly
Leo II750 kly
Sextans
Dwarf 293 kly
LMC160 kly
The Local GroupThe Milky Way and Andromeda galaxiesrule the Local Group accounting for morethan half its mass Distances from our galaxy
are given in thousands of light-years (kly) ASTRONOMY ROEN KELLY
F U N
If you examine the brightestgalaxies some 75 percent arespirals 20 percent ellipticalsand 5 percent irregularsIncluding faint dwarfs skews
the numbers to 30 percentspirals 60 percent ellipticalsand 10 percent irregulars
F A C T
ndromeda Galaxyghtly bigger than the Milky Way the Andromeda Galaxy (M31) containsme 500 billion stars Located 25 million light-years away the galaxy can bempsed with the naked eye MICHAEL STECKER
he LMC
e Large Magellanicoud (LMC) is an irregular
axy about 160000ht-years from Earth Thedish cloud at top righthe Tarantula Nebula
e largest known regionstar formation LUKE DODD
F U N
The Large and SmallMagellanic Clouds (LMCand SMC) are satellite galax-ies to the Milky Way They liedeep in the southern sky andwere not seen by Europeansuntil Magellanrsquos around-the-world voyage
F A C T
F U N
The Andromeda Galaxy mayseem a good neighbor tothe Milky Way but it wonrsquotalways be so Astronomersthink that in approximately 5billion years our two galaxieswill collide and merge
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
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Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
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Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
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Saturn
Mars
Marsrsquo ruddy appearance arisesbecause the sand on the planetrsquossurface consists largely of ironoxides mdash rust NASAJPLMSSS
Saturnrsquos rings consist of icy particlesranging in size from tiny motes tohouse-sized icebergs NASATHE HUBBLE
HERITAGE TEAM 983080STSCIAURA983081
F U N
Mars boasts the largest vol-canoes in the solar system
although theyrsquore all extinctThe biggest mdash OlympusMons mdash spans nearly 400miles and rises 13 milesabove the surrounding plain
F A C T
Earth may seem extraordinary to those who call it homebut itrsquos not a land of superlatives Earth is neither too hot
nor too cold too big nor too small Itrsquos just right in so
many ways mdash the perfect ldquoGoldilocksrdquo planet Of course
as the only known abode of life in the universe Earth
does have one major claim to being special The other planets in
the solar system leave their marks in different ways
The planets divide into two broad categories terrestrial and
jovian The small rocky terrestrial planets include Mercury
Venus Earth and Mars Mercury the closest to the Sun bakes
at temperatures up to 800deg Fahrenheit at noon But Mercuryrsquos
razor-thin atmosphere canrsquot hold heat at night the temperature
plummets far below freezing Venus most resembles Earth inmass and diameter but a thick atmosphere of carbon dioxide
has led to a runaway greenhouse effect Venusrsquo surface remains
a scorching 865deg F year-round
Earth and Mars are the water worlds of the solar system
Our home planet is the only one with liquid water at the surface
now but spacecraft observations during the past 15 years leave
no doubt that Mars once had loads of surface water Even now
Mars has permafrost and permanent polar caps of water ice
Winds up to 70 mph blow around the ubiquitous martian dust
creating shifting seasonal patterns
The jovian planets mdash Jupiter Saturn Uranus and Neptune
mdash are all gaseous behemoths They consist mostly of hydrogen
and helium the most abundant elements in the universe Jupiter
dwarfs the others It contains more than twice as much matter
as all the other planets combined All the jovian planets pos-
sess ring systems but only Saturnrsquos appears bright Its icy rings
span 170000 miles and measure just 100 feet thick Uranus
and Neptune are the true twin planets of the solar system with
nearly equal diameters masses compositions and rotations
Most scientists no longer consider small distant Pluto to be a
major planet A mixture of ice and rock this world more closely
resembles the thousands of so-called Kuiper Belt objects that
lurk beyond Neptune In 2006 astronomers demoted Pluto to a
ldquodwarf planetrdquo a category that also includes the asteroid Ceres
Planetsof thesolar system
2 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
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Planet Distance from Sun Orbital period Diameter Mass De (Earth=1) (Earth=1) (Earth=1) (wa
Mercury 039 8797 days 0383 0055 5
Venus 072 22470 days 0949 0815 5
Earth 100 36526 days 1000 1000 5
Mars 152 68698 days 0532 0107 3
Ceres 277 460 years 0075 00002 1
Jupiter 520 1186 years 11209 317832 1
Saturn 958 2946 years 9449 95159 0
Uranus 1920 8401 years 4007 14536 1
Neptune 3005 16479 years 3883 17147 1
Pluto 3948 24768 years 0187 0002 1Note Ceres and Pluto are officially considered to be dwarf planets
Solar system planets
rcurys high density meansre than half of it must be madehe heavy elements iron and
kel NASAJPLUSGS
Venus
Pluto amp Charon
Jupiter
ercury Earth
ranus Neptune
Nearly three-quarters ofEarthrsquos surface is covered withwater Itrsquos what makes our homeworld conducive to life NASA
The orbit of dwarf planet Pluto (left along with its moon Charon) brings it cto the Sun than Neptune for 20 years out of its nearly 250-year-long circuit ESA
Thick clouds blanket Venus so astronomers use radar toits surface The atmospheric pressure there is nearly 100 tithat at Earthrsquos surface NASAJPL
Jupiter is so big that it would take 11 Earths wedged side by side to cross thegiantrsquos girth and more than 1000 Earths to fill its volume NASAJPLUNIVERSITY OF AR
nusrsquo bland cloud tops maskfact that its rotation axis lies in
orbital plane so night and day atpoles last 40 years each NASAJPL
Storms rage in Neptunersquos atmos-phere as they do in the massiveatmospheres of most of the jovianplanets NASAJPL
F U N
Saturn has the lowest densityof any planet In fact if youfilled a solar-system-sized
basin with water the ringedworld would float
F A C T
F U N
Sunlight takes just eight min-utes to reach Earth but more
than four hours to cross thevoid to Neptune and Pluto
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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4 Your guide to planets stars and galaxies
After the Sun and planets therersquos not much else in the
solar system mdash certainly not in terms of mass But
in sheer number (and in a few notable instances
prominence) the small objects hold their own The
biggest of the small bodies actually outrank the small-
est planet Both Ganymede a Jupiter moon and Titan a Saturn
moon have diameters larger than Mercury More than 170
moons have been discovered orbiting the eight planets although
the vast majority are little more than flying boulders
The smaller planets tend to have fewer moons Earth has just
one which formed when an object the size of Mars struck the
proto-Earth ejecting debris that eventually coalesced Mercuryand Venus have no moons and Mars possesses just two small
ones Oddly enough Plutorsquos large moon Charon is half the diam-
eter of the dwarf planet mdash the largest ratio in the solar system
The hefty moons of the gas giants garner most of the atten-
tion Jupiterrsquos four big moons mdash Io Europa Ganymede and
Callisto mdash form a miniature solar system Io ranks as the most
volcanically active object in the solar system Europa hides an
ocean of liquid water mdash perhaps larger than all of Earthrsquos oceans
mdash beneath its frigid ice crust Giant Ganymede also may harbor
an ocean and has a surface covered with intriguing grooved ter-
rain And Callisto sports more craters than any other object in
the solar system At the top of Saturnrsquos family of moons is Titan
which possesses a significant atmosphere and methane lakes
More than half a million asteroids also inhabit the solar system
The biggest Ceres has a diameter of 600 miles Yet most are far
smaller If you add them all up asteroids donrsquot equal the weight
of Earthrsquos Moon Most asteroids circle the Sun between the orbits
of Mars and Jupiter although a few wander into Earthrsquos vicinity
Perhaps the most spectacular small bodies are comets
Billions of these ldquodirty snowballsrdquo lurk in the outer solar sys-
tem If their long looping orbits bring them close to the Sunrsquos
warmth they shed gas and dust The Sun then blows this mate-
rial back to create a long tail Although a cometrsquos nucleus may be
only a mile or two across its tail can stretch millions of miles
Small bodiesof the solarsystem
Titan
Europa
Io
Titanrsquos hazy atmosphere glows as it scatters incom-ing sunlight The atmosphere of Saturnrsquos moon isthicker than Earthrsquos and like ours contains mainlynitrogen NASAJPLSSI
Ridges crack the surface of Jupiterrsquos moonEuropa Such ridges could be sites whereslushy water erupted through the icy surfaceand then froze NASAJPL
More than 100 active volcanoes dot the surfaceof Jupiterrsquos moon Io The plumes can reach 100miles high and spread debris over thou-sands of miles NASAJPL
4 Your guide to planets stars and galaxies
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Moon Planet Distance from Orbital period Diameter Den
planet (miles) (days) (miles) (wat
Moon Earth 238900 2732 2160 3
Io Jupiter 262100 177 2263 3Europa Jupiter 417000 355 1940 3
Ganymede Jupiter 665100 715 3271 1
Callisto Jupiter 1169900 1669 2994 1
Enceladus Saturn 147900 137 313 1
Tethys Saturn 183100 189 660 1
Dione Saturn 234500 274 698 1
Rhea Saturn 327500 452 949 1
Titan Saturn 759200 1595 3200 1
Iapetus Saturn 2212600 7933 913 1
Ariel Uranus 118600 252 719 1
Umbriel Uranus 165300 414 727 1
Titania Uranus 271100 871 980 1
Oberon Uranus 362600 1346 946 1
Triton Neptune 220400 588 1681 2Charon Pluto 12200 639 753 1
Major moons
ErosHale-Bopp
Callisto
One of the brightest comets of the past40 years Hale-Bopp wowed observersin 1997 It had a nucleus 25 miles wideand a tail that stretched more than 100million miles BILL AND SALLY FLETCHER
Saturnrsquos icy moon Enceladus reflectsmore than 90 percent of the sunlightthat reaches it the highest percentageof any object in the solar system NASAJPL
PhobosLike most small moons in the solarsystem Marsrsquo Phobos measures just a
few miles across and has an irregularshape NASAJPLMSSS
F U N
Iorsquos active volcanoes andEuroparsquos underground oceanof liquid water both stemfrom enormous tidal forcesmdash which flex and heat themoonsrsquo interiors mdash exerted
by Jupiterrsquos massive gravityF A C T
Multi-ringed impact basins some stretching more than1000 miles formed on Jupiterrsquos Callisto when massiveimpacts left concentric fractures and faults NASAJPL
Potato-shaped asteroid Eros some 20 miles long looks likea lot of other modest-sized asteroids but this object mightone day wander dangerously close to Earth NASAJHUAPL
Triton
nceladus
eyser-like plumes eposited the darktreaks seen on Nep-unersquos moon Triton At aemperature of ndash390deg F Tritonas the coldest surface known in
he solar system NASAJPL
F U N
Saturnrsquos enigmatic outermoon Iapetus has a splitpersonality Half of its surface
appears as dark as freshly laidasphalt while the oppositehemisphere reflects as muchlight as newly fallen snow
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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Protostar
A
ll stars begin their lives in the vast clouds of gas and
dust that litter galaxies like the Milky Way A single
cloud can produce hundreds or even thousands of
stars Something triggers the cloud to start collaps-
ing mdash perhaps strong winds from a massive star ora nearby supernova explosion mdash and gravity works its magic
The cloud fragments and each pocket of material continues to
contract and heat up
The contracting star becomes stable when it starts to gener-
ate energy by nuclear fusion Four hydrogen atoms combine to
form one helium atom Because one helium weighs slightly less
than the four hydrogens combined the reaction creates energy
according to Einsteinrsquos equation E=mc2
The biggest stars contain up to about 120 times as much
material as the Sun They burn hot and use their fuel rapidly
These luminaries may have a surface temperature of 70000deg F
radiate nearly a million times the Sunrsquos light and survive onlya few million years
The Sun shines at about 10000deg F and will last some 10
billion years (itrsquos about halfway through now) The smallest
stars have 8 percent of the Sunrsquos mass and glow at only 3000deg
to 4000deg F mdash so dim that they can shine for a trillion years
Once a star exhausts its hydrogen fuel the end is nigh First
it swells into a red giant expanding to a diameter of hundreds of
millions of miles and cooling to a few thousand degrees It may
tap into more nuclear reactions converting helium to carbon
for example but eventually those
fuels run out as well Stars with up
to about eight times the Sunrsquos mass
eventually puff off their outer layers
and form glowing gas clouds known
as planetary nebulae The star itself
settles down as a white dwarf
More massive stars typically die
in supernova explosions Such explo-
sions scatter the heavy elements built
up during the starrsquos life forming
the raw material for new stars and
perhaps planets The collapsed remnant of the exploded star
becomes either a rapidly spinning neutron star or a black hole
whose gravity is so strong that not even light can escape
Stars inour galaxy
The Sun
Most people think of the Sun as the anchor of our solar system mdash andthatrsquos certainly true It contains 998 percent of all the matter in the solasystem But to astronomers the Sun has even more importance It is theonly star in the universe that appears as more than a point of lightthrough a telescope Detailed observations of the Sun led scientists tounderstand how stars shine how they radiate energy and even howhuge storms wrack their surfaces NASASOHO
The life of a Sun-like star
N49
Heavy elements forged in a massive star spreadout at thousands of milesper second in supernovaremnant N49 One daythese elements may beincluded in a new stellargeneration NASATHE HUBBLE
HERITAGE TEAM 983080STSCIAURA983081F U N
Astronomers divide stars intoseven main spectral classesGenerations of students havelearned the sequence byusing the first letters in thesentence ldquoOh Be A Fine Girl(or Guy) Kiss Merdquo
F A C T
6 Your guide to planets stars and galaxies
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Main sequencesolar-type star
Red giant Asymptotic-giant-branch star Protoplanetary nebula Planetary nebula White dw
Spectral Mass Temperature Main sequence Examples class (Sun=1) (Fahrenheit) radius (Sun=1)
O 20ndash120 greater than 55000deg 12ndash25 Zeta (ζ) Puppis
B 4ndash20 17100degndash55000deg 4ndash12 Rigel Spica
A 2ndash4 12300degndash17100deg 15ndash4 Sirius Vega
F 105ndash2 10300degndash12300deg 11ndash15 Canopus Procyon
G 08ndash105 9000degndash10300deg 085ndash11 Sun Capella
K 05ndash08 6700degndash9000deg 06ndash085 Aldebaran Arcturu
M 008ndash05 3100degndash6700deg 01ndash06 Antares Betelgeus
Star characteristics
Stars like the Sun condense out of a gaseous cloud The growing protostar develops a disk (which may form planets)and shoots out material before settling down as a main sequence star converting hydrogen to helium Once the hydro-gen runs out the star swells to a red giant and becomes unstable as an asymptotic-giant-branch star before puffing offits outer layers as a planetary nebula The starrsquos core remains as a dense white dwarf ASTRONOMY ROEN KELLY
Surface of the SunCatrsquos Eye Nebula
Cone Nebula
New stars form from clouds of gas and dust such asthe Cone Nebula Hot stars ionize the surroundinghydrogen gas which glows with a characteristic redcolor NASAESATHE ACS SCIENCE TEAM
When the Sun dies in 5 billion years it may resem-ble the symmetric Catrsquos Eye Nebula Here glowingstrands of ionized gas mark where a dying starrepeatedly shed its outer layers NASAESAHEIC
THE HUBBLE HERI TAGE TEAM 983080STSCIAURA983081
An intricate honeycomb on the Sunrsquos surface marks regionswhere heat (bright areas) rises and cooler material (dark areasinks in a process called convection ROYAL SWEDISH ACADEMY OF SCIE
F U N
To shine as brightly as it doesand nourish life on Earth theSun must convert 600 milliontons of hydrogen into heliumevery second
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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S c u t
u m - C
e n t a
u r u s
A r m
Norma Ar m
C e n t r a l b a r
S a g i t t a r i u s A
r m
P e r s e
u s A
r m
O r i o n
S p u r
Sun
The MilkyWay Galaxy
Head outside on a clear dark summerrsquos night and your
eyes will be greeted by thousands of stars All of them
belong to our galaxy as does virtually everything
else you can see with the naked eye If you let youreyes adjust to the darkness yoursquoll see a gauzy whitish
band running across the sky This is the Milky Way mdash the com-
bined light of countless stars mdash and the feature that lends its
name to our galaxy
The Milky Way is a giant barred spiral galaxy that stretches
about 120000 light-years from end to end but whose disk mea-
sures only some 1000 light-years thick The central bar extends
28000 light-years The Sun lies about halfway between the gal-
axyrsquos center and edge and revolves at approximately 150 miles
per second taking roughly 225 mil-
lion years to complete one circuit of
the galactic hubThe most obvious sights of the
galaxy are stars Astronomers esti-
mate between 200 and 400 billion
populate the Milky Way Galaxy
(most are hidden from view or
extremely faint so a precise count
isnrsquot possible)
Because the hottest brightest stars
are also short-lived mdash and the spiral
arms are the only place in the galaxy
with active star formation mdash the arms stand out The clouds
of gas and dust from which stars form also call the spiral arms
home as do the open star clusters that emerge from them
The nuclear bulge of the galaxy consists mostly of old stars
It measures about 12000 light-years across At the galaxyrsquos heart
lies a supermassive black hole that weighs approximately 4 mil-
lion Suns Surrounding the bulge and disk is a vast spherical
halo that stretches some 300000 light-years
The most prominent members of the halo are globular
clusters These ancient collections of up to a million stars each
were born at the same time as the galaxy some 12 billion to 13
billion years ago They contain few heavy elements because they
formed before supernova explosions had enriched the inter-
stellar medium with them
Structure of the Milky Way
The Pleiades M3
Star formation in Cygnus
F U N
Most naked-eye stars aremassive and highly luminousones that shine across greatdistances But this gives a dis-torted view of the galaxy as awhole In actuality cool dimM-type stars make up abouttwo-thirds of all stars in theMilky Way
F A C T
A stellar nursery in Cygnus harbors many massive young stars Invisibloptical light the DR21 complex shows up when viewed in dust-penetratinfrared radiation NASAJPL983085CALTECHA MARSTON 983080ESTECESA983081
Open star clusters like the Pleiadescontain dozens to hundreds of starsThese groups lie in our galaxyrsquos spi-ral arms and disperse over billionsof years JASON WARE
Globular star clusters have eas long as the Milky Way M3 p500000 stars in a sphere 160 years across S KAFKA AND K HONEYC
INDIANA UNIVERSITYWIYNNOAONSF
The Sun lies in the Orion Spur one of several arms and smaller appenda
where our galaxy creates stars Astronomers name the spiral arms after tconstellation where they appear prominent NASAJPL983085CALTECHR HURT 983080SSC983085CALT8 Your guide to planets stars and galaxies
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F U N
Much of the Milky Way Galaxyand its structure remain hid-den to earthbound observersbecause dust chokes the spi-ral arms Itrsquos like being in thewoods and trying to discernthe forestrsquos form
F A C T
F U N
In measuring distances inthe galaxy and the universeastronomers use a unitknown as the l ight-year Itrepresents the distance abeam of light travels in oneyear At 186000 miles persecond light traverses 59trillion miles in a year
F A C T
band of dust cuts through the Milky Way blocking light
om distant stars If not for all the dust the galaxyrsquos centerould shine brighter than the brightest star STEVE THORNTON
F U N
A century ago astronomersthought the Sun occupiedthe center of the galaxy Butcareful studies of globularclusters which orbit theMilky Wayrsquos center and tendto gather in the constellationSagittarius show we livehalfway to the edge
F A C T
F U N
How do astronomers knowa black hole resides at theMilky Wayrsquos center They havefound stars near the centralhub orbiting so fast that theymust be circling an invisibleobject containing 4 millionsolar masses
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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T
he collections of stars gas and dust known as galaxies
form the building blocks of the observable universe
Roughly 125 billion galaxies populate the cosmos and
they come in all shapes and sizes Astronomers divide
galaxies into three major categories spirals ellipticalsand irregulars A spiral has a broad disk containing clouds of gas
and dust and from two to several spiral arms a nuclear bulge of
old stars and a spherical halo that envelops both
Approximately one-third of spirals exhibit central bars mdash a
symmetric concentration of stars and sometimes gas and dust
that crosses the nucleus and connects with the outer spiral arms
(Recent studies show the Milky Way possesses a significant bar)
The diameters of spirals range from roughly 20000 to more
than 100000 light-years and they contain anywhere from sev-
eral billion to several hundred billion stars
Elliptical galaxies appear spherical or flattened in shape
They possess little of the gas and dust seen in the disks of spiralgalaxies so they donrsquot generate any new stars Ellipticals show
the widest range in size of any galaxy type Giant ellipticals can
span 1 million light-years and contain several trillion stars
dwarf ellipticals may be only a few thousand light-years across
and have millions of stars An important intermediate type of
galaxy has a disk like a spiral galaxy but contains no gas or dust
Astronomers call this type of galaxy a lenticular
Irregular galaxies donrsquot show any symmetry or organized
spiral structure The category exists basically as a catchall for
galaxies that donrsquot fit either the spiral or elliptical classification
Irregulars can be big containing up to 100 billion stars or as
small as dwarf ellipticals Astronomers think most irregulars
result from the collisions or mergers of two or more galaxies
The gravitational interactions disrupt normal spiral or elliptical
structure leaving behind a chaotic appearance
Most galaxies belong to groups with dozens of members or
to clusters with up to thousands of members The Milky Way
joins with the slightly larger Andromeda Galaxy to form the
cornerstones of the Local Group a collection of roughly 50 gal-
axies that spans several million light-years The vast majority of
Local Group galaxies are dwarf ellipticals and irregulars Small
groups generally have a few dozen member galaxies but clusters
can contain several thousand galaxies The Virgo cluster located
50 million light-years away is the nearest large cluster to Earth
Galaxies
in theuniverse
The Mice
NGC 4414
Multiple spiral arms wind out from the nucleus ofNGC 4414 Young blue stars throng the armswhile older redder stars populate the nuclearbulge NASATHE HUBBLE HERITAGE TEAM 983080STSCIAURA983081
The ldquoMicerdquo are two spiral galaxies in the process of merging Gravity haspulled material out of each to form long tails while compressed gas cloufuel new star formation NASAESATHE ACS SCIENCE TEAM
M87
A high-speed jet shoots from the heart of the giant
elliptical galaxy M87 (upper left) in the Virgo cluster A black hole of some3 billion solar masses drives this activity NASATHE HUBBLE HERITAGE TEAM 983080STSCIAUR
Giant elliptical galaxies M84 (left) and M86 (right)each contain a trillion stars These two dominate the central regionof the nearby Virgo cluster a collection of some 2000 galaxies NOAOAURA
M84 amp M86
10 Your guide to planets stars and galaxies
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NGC 68221760 kly
Ursa Minor Dwarf 225 kly
Draco Dwarf
248 kly MilkyWay
SMC189 kly
NGC 1852020 kly
NGC 1472460 kly
Andromeda Galax
2510 kly
Pinwheel Galaxy (M33)
2770 kly
Leo I818 kly
Leo II750 kly
Sextans
Dwarf 293 kly
LMC160 kly
The Local GroupThe Milky Way and Andromeda galaxiesrule the Local Group accounting for morethan half its mass Distances from our galaxy
are given in thousands of light-years (kly) ASTRONOMY ROEN KELLY
F U N
If you examine the brightestgalaxies some 75 percent arespirals 20 percent ellipticalsand 5 percent irregularsIncluding faint dwarfs skews
the numbers to 30 percentspirals 60 percent ellipticalsand 10 percent irregulars
F A C T
ndromeda Galaxyghtly bigger than the Milky Way the Andromeda Galaxy (M31) containsme 500 billion stars Located 25 million light-years away the galaxy can bempsed with the naked eye MICHAEL STECKER
he LMC
e Large Magellanicoud (LMC) is an irregular
axy about 160000ht-years from Earth Thedish cloud at top righthe Tarantula Nebula
e largest known regionstar formation LUKE DODD
F U N
The Large and SmallMagellanic Clouds (LMCand SMC) are satellite galax-ies to the Milky Way They liedeep in the southern sky andwere not seen by Europeansuntil Magellanrsquos around-the-world voyage
F A C T
F U N
The Andromeda Galaxy mayseem a good neighbor tothe Milky Way but it wonrsquotalways be so Astronomersthink that in approximately 5billion years our two galaxieswill collide and merge
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
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Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
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Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
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Planet Distance from Sun Orbital period Diameter Mass De (Earth=1) (Earth=1) (Earth=1) (wa
Mercury 039 8797 days 0383 0055 5
Venus 072 22470 days 0949 0815 5
Earth 100 36526 days 1000 1000 5
Mars 152 68698 days 0532 0107 3
Ceres 277 460 years 0075 00002 1
Jupiter 520 1186 years 11209 317832 1
Saturn 958 2946 years 9449 95159 0
Uranus 1920 8401 years 4007 14536 1
Neptune 3005 16479 years 3883 17147 1
Pluto 3948 24768 years 0187 0002 1Note Ceres and Pluto are officially considered to be dwarf planets
Solar system planets
rcurys high density meansre than half of it must be madehe heavy elements iron and
kel NASAJPLUSGS
Venus
Pluto amp Charon
Jupiter
ercury Earth
ranus Neptune
Nearly three-quarters ofEarthrsquos surface is covered withwater Itrsquos what makes our homeworld conducive to life NASA
The orbit of dwarf planet Pluto (left along with its moon Charon) brings it cto the Sun than Neptune for 20 years out of its nearly 250-year-long circuit ESA
Thick clouds blanket Venus so astronomers use radar toits surface The atmospheric pressure there is nearly 100 tithat at Earthrsquos surface NASAJPL
Jupiter is so big that it would take 11 Earths wedged side by side to cross thegiantrsquos girth and more than 1000 Earths to fill its volume NASAJPLUNIVERSITY OF AR
nusrsquo bland cloud tops maskfact that its rotation axis lies in
orbital plane so night and day atpoles last 40 years each NASAJPL
Storms rage in Neptunersquos atmos-phere as they do in the massiveatmospheres of most of the jovianplanets NASAJPL
F U N
Saturn has the lowest densityof any planet In fact if youfilled a solar-system-sized
basin with water the ringedworld would float
F A C T
F U N
Sunlight takes just eight min-utes to reach Earth but more
than four hours to cross thevoid to Neptune and Pluto
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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4 Your guide to planets stars and galaxies
After the Sun and planets therersquos not much else in the
solar system mdash certainly not in terms of mass But
in sheer number (and in a few notable instances
prominence) the small objects hold their own The
biggest of the small bodies actually outrank the small-
est planet Both Ganymede a Jupiter moon and Titan a Saturn
moon have diameters larger than Mercury More than 170
moons have been discovered orbiting the eight planets although
the vast majority are little more than flying boulders
The smaller planets tend to have fewer moons Earth has just
one which formed when an object the size of Mars struck the
proto-Earth ejecting debris that eventually coalesced Mercuryand Venus have no moons and Mars possesses just two small
ones Oddly enough Plutorsquos large moon Charon is half the diam-
eter of the dwarf planet mdash the largest ratio in the solar system
The hefty moons of the gas giants garner most of the atten-
tion Jupiterrsquos four big moons mdash Io Europa Ganymede and
Callisto mdash form a miniature solar system Io ranks as the most
volcanically active object in the solar system Europa hides an
ocean of liquid water mdash perhaps larger than all of Earthrsquos oceans
mdash beneath its frigid ice crust Giant Ganymede also may harbor
an ocean and has a surface covered with intriguing grooved ter-
rain And Callisto sports more craters than any other object in
the solar system At the top of Saturnrsquos family of moons is Titan
which possesses a significant atmosphere and methane lakes
More than half a million asteroids also inhabit the solar system
The biggest Ceres has a diameter of 600 miles Yet most are far
smaller If you add them all up asteroids donrsquot equal the weight
of Earthrsquos Moon Most asteroids circle the Sun between the orbits
of Mars and Jupiter although a few wander into Earthrsquos vicinity
Perhaps the most spectacular small bodies are comets
Billions of these ldquodirty snowballsrdquo lurk in the outer solar sys-
tem If their long looping orbits bring them close to the Sunrsquos
warmth they shed gas and dust The Sun then blows this mate-
rial back to create a long tail Although a cometrsquos nucleus may be
only a mile or two across its tail can stretch millions of miles
Small bodiesof the solarsystem
Titan
Europa
Io
Titanrsquos hazy atmosphere glows as it scatters incom-ing sunlight The atmosphere of Saturnrsquos moon isthicker than Earthrsquos and like ours contains mainlynitrogen NASAJPLSSI
Ridges crack the surface of Jupiterrsquos moonEuropa Such ridges could be sites whereslushy water erupted through the icy surfaceand then froze NASAJPL
More than 100 active volcanoes dot the surfaceof Jupiterrsquos moon Io The plumes can reach 100miles high and spread debris over thou-sands of miles NASAJPL
4 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
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Moon Planet Distance from Orbital period Diameter Den
planet (miles) (days) (miles) (wat
Moon Earth 238900 2732 2160 3
Io Jupiter 262100 177 2263 3Europa Jupiter 417000 355 1940 3
Ganymede Jupiter 665100 715 3271 1
Callisto Jupiter 1169900 1669 2994 1
Enceladus Saturn 147900 137 313 1
Tethys Saturn 183100 189 660 1
Dione Saturn 234500 274 698 1
Rhea Saturn 327500 452 949 1
Titan Saturn 759200 1595 3200 1
Iapetus Saturn 2212600 7933 913 1
Ariel Uranus 118600 252 719 1
Umbriel Uranus 165300 414 727 1
Titania Uranus 271100 871 980 1
Oberon Uranus 362600 1346 946 1
Triton Neptune 220400 588 1681 2Charon Pluto 12200 639 753 1
Major moons
ErosHale-Bopp
Callisto
One of the brightest comets of the past40 years Hale-Bopp wowed observersin 1997 It had a nucleus 25 miles wideand a tail that stretched more than 100million miles BILL AND SALLY FLETCHER
Saturnrsquos icy moon Enceladus reflectsmore than 90 percent of the sunlightthat reaches it the highest percentageof any object in the solar system NASAJPL
PhobosLike most small moons in the solarsystem Marsrsquo Phobos measures just a
few miles across and has an irregularshape NASAJPLMSSS
F U N
Iorsquos active volcanoes andEuroparsquos underground oceanof liquid water both stemfrom enormous tidal forcesmdash which flex and heat themoonsrsquo interiors mdash exerted
by Jupiterrsquos massive gravityF A C T
Multi-ringed impact basins some stretching more than1000 miles formed on Jupiterrsquos Callisto when massiveimpacts left concentric fractures and faults NASAJPL
Potato-shaped asteroid Eros some 20 miles long looks likea lot of other modest-sized asteroids but this object mightone day wander dangerously close to Earth NASAJHUAPL
Triton
nceladus
eyser-like plumes eposited the darktreaks seen on Nep-unersquos moon Triton At aemperature of ndash390deg F Tritonas the coldest surface known in
he solar system NASAJPL
F U N
Saturnrsquos enigmatic outermoon Iapetus has a splitpersonality Half of its surface
appears as dark as freshly laidasphalt while the oppositehemisphere reflects as muchlight as newly fallen snow
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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Protostar
A
ll stars begin their lives in the vast clouds of gas and
dust that litter galaxies like the Milky Way A single
cloud can produce hundreds or even thousands of
stars Something triggers the cloud to start collaps-
ing mdash perhaps strong winds from a massive star ora nearby supernova explosion mdash and gravity works its magic
The cloud fragments and each pocket of material continues to
contract and heat up
The contracting star becomes stable when it starts to gener-
ate energy by nuclear fusion Four hydrogen atoms combine to
form one helium atom Because one helium weighs slightly less
than the four hydrogens combined the reaction creates energy
according to Einsteinrsquos equation E=mc2
The biggest stars contain up to about 120 times as much
material as the Sun They burn hot and use their fuel rapidly
These luminaries may have a surface temperature of 70000deg F
radiate nearly a million times the Sunrsquos light and survive onlya few million years
The Sun shines at about 10000deg F and will last some 10
billion years (itrsquos about halfway through now) The smallest
stars have 8 percent of the Sunrsquos mass and glow at only 3000deg
to 4000deg F mdash so dim that they can shine for a trillion years
Once a star exhausts its hydrogen fuel the end is nigh First
it swells into a red giant expanding to a diameter of hundreds of
millions of miles and cooling to a few thousand degrees It may
tap into more nuclear reactions converting helium to carbon
for example but eventually those
fuels run out as well Stars with up
to about eight times the Sunrsquos mass
eventually puff off their outer layers
and form glowing gas clouds known
as planetary nebulae The star itself
settles down as a white dwarf
More massive stars typically die
in supernova explosions Such explo-
sions scatter the heavy elements built
up during the starrsquos life forming
the raw material for new stars and
perhaps planets The collapsed remnant of the exploded star
becomes either a rapidly spinning neutron star or a black hole
whose gravity is so strong that not even light can escape
Stars inour galaxy
The Sun
Most people think of the Sun as the anchor of our solar system mdash andthatrsquos certainly true It contains 998 percent of all the matter in the solasystem But to astronomers the Sun has even more importance It is theonly star in the universe that appears as more than a point of lightthrough a telescope Detailed observations of the Sun led scientists tounderstand how stars shine how they radiate energy and even howhuge storms wrack their surfaces NASASOHO
The life of a Sun-like star
N49
Heavy elements forged in a massive star spreadout at thousands of milesper second in supernovaremnant N49 One daythese elements may beincluded in a new stellargeneration NASATHE HUBBLE
HERITAGE TEAM 983080STSCIAURA983081F U N
Astronomers divide stars intoseven main spectral classesGenerations of students havelearned the sequence byusing the first letters in thesentence ldquoOh Be A Fine Girl(or Guy) Kiss Merdquo
F A C T
6 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
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Main sequencesolar-type star
Red giant Asymptotic-giant-branch star Protoplanetary nebula Planetary nebula White dw
Spectral Mass Temperature Main sequence Examples class (Sun=1) (Fahrenheit) radius (Sun=1)
O 20ndash120 greater than 55000deg 12ndash25 Zeta (ζ) Puppis
B 4ndash20 17100degndash55000deg 4ndash12 Rigel Spica
A 2ndash4 12300degndash17100deg 15ndash4 Sirius Vega
F 105ndash2 10300degndash12300deg 11ndash15 Canopus Procyon
G 08ndash105 9000degndash10300deg 085ndash11 Sun Capella
K 05ndash08 6700degndash9000deg 06ndash085 Aldebaran Arcturu
M 008ndash05 3100degndash6700deg 01ndash06 Antares Betelgeus
Star characteristics
Stars like the Sun condense out of a gaseous cloud The growing protostar develops a disk (which may form planets)and shoots out material before settling down as a main sequence star converting hydrogen to helium Once the hydro-gen runs out the star swells to a red giant and becomes unstable as an asymptotic-giant-branch star before puffing offits outer layers as a planetary nebula The starrsquos core remains as a dense white dwarf ASTRONOMY ROEN KELLY
Surface of the SunCatrsquos Eye Nebula
Cone Nebula
New stars form from clouds of gas and dust such asthe Cone Nebula Hot stars ionize the surroundinghydrogen gas which glows with a characteristic redcolor NASAESATHE ACS SCIENCE TEAM
When the Sun dies in 5 billion years it may resem-ble the symmetric Catrsquos Eye Nebula Here glowingstrands of ionized gas mark where a dying starrepeatedly shed its outer layers NASAESAHEIC
THE HUBBLE HERI TAGE TEAM 983080STSCIAURA983081
An intricate honeycomb on the Sunrsquos surface marks regionswhere heat (bright areas) rises and cooler material (dark areasinks in a process called convection ROYAL SWEDISH ACADEMY OF SCIE
F U N
To shine as brightly as it doesand nourish life on Earth theSun must convert 600 milliontons of hydrogen into heliumevery second
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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S c u t
u m - C
e n t a
u r u s
A r m
Norma Ar m
C e n t r a l b a r
S a g i t t a r i u s A
r m
P e r s e
u s A
r m
O r i o n
S p u r
Sun
The MilkyWay Galaxy
Head outside on a clear dark summerrsquos night and your
eyes will be greeted by thousands of stars All of them
belong to our galaxy as does virtually everything
else you can see with the naked eye If you let youreyes adjust to the darkness yoursquoll see a gauzy whitish
band running across the sky This is the Milky Way mdash the com-
bined light of countless stars mdash and the feature that lends its
name to our galaxy
The Milky Way is a giant barred spiral galaxy that stretches
about 120000 light-years from end to end but whose disk mea-
sures only some 1000 light-years thick The central bar extends
28000 light-years The Sun lies about halfway between the gal-
axyrsquos center and edge and revolves at approximately 150 miles
per second taking roughly 225 mil-
lion years to complete one circuit of
the galactic hubThe most obvious sights of the
galaxy are stars Astronomers esti-
mate between 200 and 400 billion
populate the Milky Way Galaxy
(most are hidden from view or
extremely faint so a precise count
isnrsquot possible)
Because the hottest brightest stars
are also short-lived mdash and the spiral
arms are the only place in the galaxy
with active star formation mdash the arms stand out The clouds
of gas and dust from which stars form also call the spiral arms
home as do the open star clusters that emerge from them
The nuclear bulge of the galaxy consists mostly of old stars
It measures about 12000 light-years across At the galaxyrsquos heart
lies a supermassive black hole that weighs approximately 4 mil-
lion Suns Surrounding the bulge and disk is a vast spherical
halo that stretches some 300000 light-years
The most prominent members of the halo are globular
clusters These ancient collections of up to a million stars each
were born at the same time as the galaxy some 12 billion to 13
billion years ago They contain few heavy elements because they
formed before supernova explosions had enriched the inter-
stellar medium with them
Structure of the Milky Way
The Pleiades M3
Star formation in Cygnus
F U N
Most naked-eye stars aremassive and highly luminousones that shine across greatdistances But this gives a dis-torted view of the galaxy as awhole In actuality cool dimM-type stars make up abouttwo-thirds of all stars in theMilky Way
F A C T
A stellar nursery in Cygnus harbors many massive young stars Invisibloptical light the DR21 complex shows up when viewed in dust-penetratinfrared radiation NASAJPL983085CALTECHA MARSTON 983080ESTECESA983081
Open star clusters like the Pleiadescontain dozens to hundreds of starsThese groups lie in our galaxyrsquos spi-ral arms and disperse over billionsof years JASON WARE
Globular star clusters have eas long as the Milky Way M3 p500000 stars in a sphere 160 years across S KAFKA AND K HONEYC
INDIANA UNIVERSITYWIYNNOAONSF
The Sun lies in the Orion Spur one of several arms and smaller appenda
where our galaxy creates stars Astronomers name the spiral arms after tconstellation where they appear prominent NASAJPL983085CALTECHR HURT 983080SSC983085CALT8 Your guide to planets stars and galaxies
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F U N
Much of the Milky Way Galaxyand its structure remain hid-den to earthbound observersbecause dust chokes the spi-ral arms Itrsquos like being in thewoods and trying to discernthe forestrsquos form
F A C T
F U N
In measuring distances inthe galaxy and the universeastronomers use a unitknown as the l ight-year Itrepresents the distance abeam of light travels in oneyear At 186000 miles persecond light traverses 59trillion miles in a year
F A C T
band of dust cuts through the Milky Way blocking light
om distant stars If not for all the dust the galaxyrsquos centerould shine brighter than the brightest star STEVE THORNTON
F U N
A century ago astronomersthought the Sun occupiedthe center of the galaxy Butcareful studies of globularclusters which orbit theMilky Wayrsquos center and tendto gather in the constellationSagittarius show we livehalfway to the edge
F A C T
F U N
How do astronomers knowa black hole resides at theMilky Wayrsquos center They havefound stars near the centralhub orbiting so fast that theymust be circling an invisibleobject containing 4 millionsolar masses
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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T
he collections of stars gas and dust known as galaxies
form the building blocks of the observable universe
Roughly 125 billion galaxies populate the cosmos and
they come in all shapes and sizes Astronomers divide
galaxies into three major categories spirals ellipticalsand irregulars A spiral has a broad disk containing clouds of gas
and dust and from two to several spiral arms a nuclear bulge of
old stars and a spherical halo that envelops both
Approximately one-third of spirals exhibit central bars mdash a
symmetric concentration of stars and sometimes gas and dust
that crosses the nucleus and connects with the outer spiral arms
(Recent studies show the Milky Way possesses a significant bar)
The diameters of spirals range from roughly 20000 to more
than 100000 light-years and they contain anywhere from sev-
eral billion to several hundred billion stars
Elliptical galaxies appear spherical or flattened in shape
They possess little of the gas and dust seen in the disks of spiralgalaxies so they donrsquot generate any new stars Ellipticals show
the widest range in size of any galaxy type Giant ellipticals can
span 1 million light-years and contain several trillion stars
dwarf ellipticals may be only a few thousand light-years across
and have millions of stars An important intermediate type of
galaxy has a disk like a spiral galaxy but contains no gas or dust
Astronomers call this type of galaxy a lenticular
Irregular galaxies donrsquot show any symmetry or organized
spiral structure The category exists basically as a catchall for
galaxies that donrsquot fit either the spiral or elliptical classification
Irregulars can be big containing up to 100 billion stars or as
small as dwarf ellipticals Astronomers think most irregulars
result from the collisions or mergers of two or more galaxies
The gravitational interactions disrupt normal spiral or elliptical
structure leaving behind a chaotic appearance
Most galaxies belong to groups with dozens of members or
to clusters with up to thousands of members The Milky Way
joins with the slightly larger Andromeda Galaxy to form the
cornerstones of the Local Group a collection of roughly 50 gal-
axies that spans several million light-years The vast majority of
Local Group galaxies are dwarf ellipticals and irregulars Small
groups generally have a few dozen member galaxies but clusters
can contain several thousand galaxies The Virgo cluster located
50 million light-years away is the nearest large cluster to Earth
Galaxies
in theuniverse
The Mice
NGC 4414
Multiple spiral arms wind out from the nucleus ofNGC 4414 Young blue stars throng the armswhile older redder stars populate the nuclearbulge NASATHE HUBBLE HERITAGE TEAM 983080STSCIAURA983081
The ldquoMicerdquo are two spiral galaxies in the process of merging Gravity haspulled material out of each to form long tails while compressed gas cloufuel new star formation NASAESATHE ACS SCIENCE TEAM
M87
A high-speed jet shoots from the heart of the giant
elliptical galaxy M87 (upper left) in the Virgo cluster A black hole of some3 billion solar masses drives this activity NASATHE HUBBLE HERITAGE TEAM 983080STSCIAUR
Giant elliptical galaxies M84 (left) and M86 (right)each contain a trillion stars These two dominate the central regionof the nearby Virgo cluster a collection of some 2000 galaxies NOAOAURA
M84 amp M86
10 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
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NGC 68221760 kly
Ursa Minor Dwarf 225 kly
Draco Dwarf
248 kly MilkyWay
SMC189 kly
NGC 1852020 kly
NGC 1472460 kly
Andromeda Galax
2510 kly
Pinwheel Galaxy (M33)
2770 kly
Leo I818 kly
Leo II750 kly
Sextans
Dwarf 293 kly
LMC160 kly
The Local GroupThe Milky Way and Andromeda galaxiesrule the Local Group accounting for morethan half its mass Distances from our galaxy
are given in thousands of light-years (kly) ASTRONOMY ROEN KELLY
F U N
If you examine the brightestgalaxies some 75 percent arespirals 20 percent ellipticalsand 5 percent irregularsIncluding faint dwarfs skews
the numbers to 30 percentspirals 60 percent ellipticalsand 10 percent irregulars
F A C T
ndromeda Galaxyghtly bigger than the Milky Way the Andromeda Galaxy (M31) containsme 500 billion stars Located 25 million light-years away the galaxy can bempsed with the naked eye MICHAEL STECKER
he LMC
e Large Magellanicoud (LMC) is an irregular
axy about 160000ht-years from Earth Thedish cloud at top righthe Tarantula Nebula
e largest known regionstar formation LUKE DODD
F U N
The Large and SmallMagellanic Clouds (LMCand SMC) are satellite galax-ies to the Milky Way They liedeep in the southern sky andwere not seen by Europeansuntil Magellanrsquos around-the-world voyage
F A C T
F U N
The Andromeda Galaxy mayseem a good neighbor tothe Milky Way but it wonrsquotalways be so Astronomersthink that in approximately 5billion years our two galaxieswill collide and merge
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
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Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
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Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 414
4 Your guide to planets stars and galaxies
After the Sun and planets therersquos not much else in the
solar system mdash certainly not in terms of mass But
in sheer number (and in a few notable instances
prominence) the small objects hold their own The
biggest of the small bodies actually outrank the small-
est planet Both Ganymede a Jupiter moon and Titan a Saturn
moon have diameters larger than Mercury More than 170
moons have been discovered orbiting the eight planets although
the vast majority are little more than flying boulders
The smaller planets tend to have fewer moons Earth has just
one which formed when an object the size of Mars struck the
proto-Earth ejecting debris that eventually coalesced Mercuryand Venus have no moons and Mars possesses just two small
ones Oddly enough Plutorsquos large moon Charon is half the diam-
eter of the dwarf planet mdash the largest ratio in the solar system
The hefty moons of the gas giants garner most of the atten-
tion Jupiterrsquos four big moons mdash Io Europa Ganymede and
Callisto mdash form a miniature solar system Io ranks as the most
volcanically active object in the solar system Europa hides an
ocean of liquid water mdash perhaps larger than all of Earthrsquos oceans
mdash beneath its frigid ice crust Giant Ganymede also may harbor
an ocean and has a surface covered with intriguing grooved ter-
rain And Callisto sports more craters than any other object in
the solar system At the top of Saturnrsquos family of moons is Titan
which possesses a significant atmosphere and methane lakes
More than half a million asteroids also inhabit the solar system
The biggest Ceres has a diameter of 600 miles Yet most are far
smaller If you add them all up asteroids donrsquot equal the weight
of Earthrsquos Moon Most asteroids circle the Sun between the orbits
of Mars and Jupiter although a few wander into Earthrsquos vicinity
Perhaps the most spectacular small bodies are comets
Billions of these ldquodirty snowballsrdquo lurk in the outer solar sys-
tem If their long looping orbits bring them close to the Sunrsquos
warmth they shed gas and dust The Sun then blows this mate-
rial back to create a long tail Although a cometrsquos nucleus may be
only a mile or two across its tail can stretch millions of miles
Small bodiesof the solarsystem
Titan
Europa
Io
Titanrsquos hazy atmosphere glows as it scatters incom-ing sunlight The atmosphere of Saturnrsquos moon isthicker than Earthrsquos and like ours contains mainlynitrogen NASAJPLSSI
Ridges crack the surface of Jupiterrsquos moonEuropa Such ridges could be sites whereslushy water erupted through the icy surfaceand then froze NASAJPL
More than 100 active volcanoes dot the surfaceof Jupiterrsquos moon Io The plumes can reach 100miles high and spread debris over thou-sands of miles NASAJPL
4 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
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Moon Planet Distance from Orbital period Diameter Den
planet (miles) (days) (miles) (wat
Moon Earth 238900 2732 2160 3
Io Jupiter 262100 177 2263 3Europa Jupiter 417000 355 1940 3
Ganymede Jupiter 665100 715 3271 1
Callisto Jupiter 1169900 1669 2994 1
Enceladus Saturn 147900 137 313 1
Tethys Saturn 183100 189 660 1
Dione Saturn 234500 274 698 1
Rhea Saturn 327500 452 949 1
Titan Saturn 759200 1595 3200 1
Iapetus Saturn 2212600 7933 913 1
Ariel Uranus 118600 252 719 1
Umbriel Uranus 165300 414 727 1
Titania Uranus 271100 871 980 1
Oberon Uranus 362600 1346 946 1
Triton Neptune 220400 588 1681 2Charon Pluto 12200 639 753 1
Major moons
ErosHale-Bopp
Callisto
One of the brightest comets of the past40 years Hale-Bopp wowed observersin 1997 It had a nucleus 25 miles wideand a tail that stretched more than 100million miles BILL AND SALLY FLETCHER
Saturnrsquos icy moon Enceladus reflectsmore than 90 percent of the sunlightthat reaches it the highest percentageof any object in the solar system NASAJPL
PhobosLike most small moons in the solarsystem Marsrsquo Phobos measures just a
few miles across and has an irregularshape NASAJPLMSSS
F U N
Iorsquos active volcanoes andEuroparsquos underground oceanof liquid water both stemfrom enormous tidal forcesmdash which flex and heat themoonsrsquo interiors mdash exerted
by Jupiterrsquos massive gravityF A C T
Multi-ringed impact basins some stretching more than1000 miles formed on Jupiterrsquos Callisto when massiveimpacts left concentric fractures and faults NASAJPL
Potato-shaped asteroid Eros some 20 miles long looks likea lot of other modest-sized asteroids but this object mightone day wander dangerously close to Earth NASAJHUAPL
Triton
nceladus
eyser-like plumes eposited the darktreaks seen on Nep-unersquos moon Triton At aemperature of ndash390deg F Tritonas the coldest surface known in
he solar system NASAJPL
F U N
Saturnrsquos enigmatic outermoon Iapetus has a splitpersonality Half of its surface
appears as dark as freshly laidasphalt while the oppositehemisphere reflects as muchlight as newly fallen snow
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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Protostar
A
ll stars begin their lives in the vast clouds of gas and
dust that litter galaxies like the Milky Way A single
cloud can produce hundreds or even thousands of
stars Something triggers the cloud to start collaps-
ing mdash perhaps strong winds from a massive star ora nearby supernova explosion mdash and gravity works its magic
The cloud fragments and each pocket of material continues to
contract and heat up
The contracting star becomes stable when it starts to gener-
ate energy by nuclear fusion Four hydrogen atoms combine to
form one helium atom Because one helium weighs slightly less
than the four hydrogens combined the reaction creates energy
according to Einsteinrsquos equation E=mc2
The biggest stars contain up to about 120 times as much
material as the Sun They burn hot and use their fuel rapidly
These luminaries may have a surface temperature of 70000deg F
radiate nearly a million times the Sunrsquos light and survive onlya few million years
The Sun shines at about 10000deg F and will last some 10
billion years (itrsquos about halfway through now) The smallest
stars have 8 percent of the Sunrsquos mass and glow at only 3000deg
to 4000deg F mdash so dim that they can shine for a trillion years
Once a star exhausts its hydrogen fuel the end is nigh First
it swells into a red giant expanding to a diameter of hundreds of
millions of miles and cooling to a few thousand degrees It may
tap into more nuclear reactions converting helium to carbon
for example but eventually those
fuels run out as well Stars with up
to about eight times the Sunrsquos mass
eventually puff off their outer layers
and form glowing gas clouds known
as planetary nebulae The star itself
settles down as a white dwarf
More massive stars typically die
in supernova explosions Such explo-
sions scatter the heavy elements built
up during the starrsquos life forming
the raw material for new stars and
perhaps planets The collapsed remnant of the exploded star
becomes either a rapidly spinning neutron star or a black hole
whose gravity is so strong that not even light can escape
Stars inour galaxy
The Sun
Most people think of the Sun as the anchor of our solar system mdash andthatrsquos certainly true It contains 998 percent of all the matter in the solasystem But to astronomers the Sun has even more importance It is theonly star in the universe that appears as more than a point of lightthrough a telescope Detailed observations of the Sun led scientists tounderstand how stars shine how they radiate energy and even howhuge storms wrack their surfaces NASASOHO
The life of a Sun-like star
N49
Heavy elements forged in a massive star spreadout at thousands of milesper second in supernovaremnant N49 One daythese elements may beincluded in a new stellargeneration NASATHE HUBBLE
HERITAGE TEAM 983080STSCIAURA983081F U N
Astronomers divide stars intoseven main spectral classesGenerations of students havelearned the sequence byusing the first letters in thesentence ldquoOh Be A Fine Girl(or Guy) Kiss Merdquo
F A C T
6 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 714
Main sequencesolar-type star
Red giant Asymptotic-giant-branch star Protoplanetary nebula Planetary nebula White dw
Spectral Mass Temperature Main sequence Examples class (Sun=1) (Fahrenheit) radius (Sun=1)
O 20ndash120 greater than 55000deg 12ndash25 Zeta (ζ) Puppis
B 4ndash20 17100degndash55000deg 4ndash12 Rigel Spica
A 2ndash4 12300degndash17100deg 15ndash4 Sirius Vega
F 105ndash2 10300degndash12300deg 11ndash15 Canopus Procyon
G 08ndash105 9000degndash10300deg 085ndash11 Sun Capella
K 05ndash08 6700degndash9000deg 06ndash085 Aldebaran Arcturu
M 008ndash05 3100degndash6700deg 01ndash06 Antares Betelgeus
Star characteristics
Stars like the Sun condense out of a gaseous cloud The growing protostar develops a disk (which may form planets)and shoots out material before settling down as a main sequence star converting hydrogen to helium Once the hydro-gen runs out the star swells to a red giant and becomes unstable as an asymptotic-giant-branch star before puffing offits outer layers as a planetary nebula The starrsquos core remains as a dense white dwarf ASTRONOMY ROEN KELLY
Surface of the SunCatrsquos Eye Nebula
Cone Nebula
New stars form from clouds of gas and dust such asthe Cone Nebula Hot stars ionize the surroundinghydrogen gas which glows with a characteristic redcolor NASAESATHE ACS SCIENCE TEAM
When the Sun dies in 5 billion years it may resem-ble the symmetric Catrsquos Eye Nebula Here glowingstrands of ionized gas mark where a dying starrepeatedly shed its outer layers NASAESAHEIC
THE HUBBLE HERI TAGE TEAM 983080STSCIAURA983081
An intricate honeycomb on the Sunrsquos surface marks regionswhere heat (bright areas) rises and cooler material (dark areasinks in a process called convection ROYAL SWEDISH ACADEMY OF SCIE
F U N
To shine as brightly as it doesand nourish life on Earth theSun must convert 600 milliontons of hydrogen into heliumevery second
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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S c u t
u m - C
e n t a
u r u s
A r m
Norma Ar m
C e n t r a l b a r
S a g i t t a r i u s A
r m
P e r s e
u s A
r m
O r i o n
S p u r
Sun
The MilkyWay Galaxy
Head outside on a clear dark summerrsquos night and your
eyes will be greeted by thousands of stars All of them
belong to our galaxy as does virtually everything
else you can see with the naked eye If you let youreyes adjust to the darkness yoursquoll see a gauzy whitish
band running across the sky This is the Milky Way mdash the com-
bined light of countless stars mdash and the feature that lends its
name to our galaxy
The Milky Way is a giant barred spiral galaxy that stretches
about 120000 light-years from end to end but whose disk mea-
sures only some 1000 light-years thick The central bar extends
28000 light-years The Sun lies about halfway between the gal-
axyrsquos center and edge and revolves at approximately 150 miles
per second taking roughly 225 mil-
lion years to complete one circuit of
the galactic hubThe most obvious sights of the
galaxy are stars Astronomers esti-
mate between 200 and 400 billion
populate the Milky Way Galaxy
(most are hidden from view or
extremely faint so a precise count
isnrsquot possible)
Because the hottest brightest stars
are also short-lived mdash and the spiral
arms are the only place in the galaxy
with active star formation mdash the arms stand out The clouds
of gas and dust from which stars form also call the spiral arms
home as do the open star clusters that emerge from them
The nuclear bulge of the galaxy consists mostly of old stars
It measures about 12000 light-years across At the galaxyrsquos heart
lies a supermassive black hole that weighs approximately 4 mil-
lion Suns Surrounding the bulge and disk is a vast spherical
halo that stretches some 300000 light-years
The most prominent members of the halo are globular
clusters These ancient collections of up to a million stars each
were born at the same time as the galaxy some 12 billion to 13
billion years ago They contain few heavy elements because they
formed before supernova explosions had enriched the inter-
stellar medium with them
Structure of the Milky Way
The Pleiades M3
Star formation in Cygnus
F U N
Most naked-eye stars aremassive and highly luminousones that shine across greatdistances But this gives a dis-torted view of the galaxy as awhole In actuality cool dimM-type stars make up abouttwo-thirds of all stars in theMilky Way
F A C T
A stellar nursery in Cygnus harbors many massive young stars Invisibloptical light the DR21 complex shows up when viewed in dust-penetratinfrared radiation NASAJPL983085CALTECHA MARSTON 983080ESTECESA983081
Open star clusters like the Pleiadescontain dozens to hundreds of starsThese groups lie in our galaxyrsquos spi-ral arms and disperse over billionsof years JASON WARE
Globular star clusters have eas long as the Milky Way M3 p500000 stars in a sphere 160 years across S KAFKA AND K HONEYC
INDIANA UNIVERSITYWIYNNOAONSF
The Sun lies in the Orion Spur one of several arms and smaller appenda
where our galaxy creates stars Astronomers name the spiral arms after tconstellation where they appear prominent NASAJPL983085CALTECHR HURT 983080SSC983085CALT8 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 914
F U N
Much of the Milky Way Galaxyand its structure remain hid-den to earthbound observersbecause dust chokes the spi-ral arms Itrsquos like being in thewoods and trying to discernthe forestrsquos form
F A C T
F U N
In measuring distances inthe galaxy and the universeastronomers use a unitknown as the l ight-year Itrepresents the distance abeam of light travels in oneyear At 186000 miles persecond light traverses 59trillion miles in a year
F A C T
band of dust cuts through the Milky Way blocking light
om distant stars If not for all the dust the galaxyrsquos centerould shine brighter than the brightest star STEVE THORNTON
F U N
A century ago astronomersthought the Sun occupiedthe center of the galaxy Butcareful studies of globularclusters which orbit theMilky Wayrsquos center and tendto gather in the constellationSagittarius show we livehalfway to the edge
F A C T
F U N
How do astronomers knowa black hole resides at theMilky Wayrsquos center They havefound stars near the centralhub orbiting so fast that theymust be circling an invisibleobject containing 4 millionsolar masses
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1014
T
he collections of stars gas and dust known as galaxies
form the building blocks of the observable universe
Roughly 125 billion galaxies populate the cosmos and
they come in all shapes and sizes Astronomers divide
galaxies into three major categories spirals ellipticalsand irregulars A spiral has a broad disk containing clouds of gas
and dust and from two to several spiral arms a nuclear bulge of
old stars and a spherical halo that envelops both
Approximately one-third of spirals exhibit central bars mdash a
symmetric concentration of stars and sometimes gas and dust
that crosses the nucleus and connects with the outer spiral arms
(Recent studies show the Milky Way possesses a significant bar)
The diameters of spirals range from roughly 20000 to more
than 100000 light-years and they contain anywhere from sev-
eral billion to several hundred billion stars
Elliptical galaxies appear spherical or flattened in shape
They possess little of the gas and dust seen in the disks of spiralgalaxies so they donrsquot generate any new stars Ellipticals show
the widest range in size of any galaxy type Giant ellipticals can
span 1 million light-years and contain several trillion stars
dwarf ellipticals may be only a few thousand light-years across
and have millions of stars An important intermediate type of
galaxy has a disk like a spiral galaxy but contains no gas or dust
Astronomers call this type of galaxy a lenticular
Irregular galaxies donrsquot show any symmetry or organized
spiral structure The category exists basically as a catchall for
galaxies that donrsquot fit either the spiral or elliptical classification
Irregulars can be big containing up to 100 billion stars or as
small as dwarf ellipticals Astronomers think most irregulars
result from the collisions or mergers of two or more galaxies
The gravitational interactions disrupt normal spiral or elliptical
structure leaving behind a chaotic appearance
Most galaxies belong to groups with dozens of members or
to clusters with up to thousands of members The Milky Way
joins with the slightly larger Andromeda Galaxy to form the
cornerstones of the Local Group a collection of roughly 50 gal-
axies that spans several million light-years The vast majority of
Local Group galaxies are dwarf ellipticals and irregulars Small
groups generally have a few dozen member galaxies but clusters
can contain several thousand galaxies The Virgo cluster located
50 million light-years away is the nearest large cluster to Earth
Galaxies
in theuniverse
The Mice
NGC 4414
Multiple spiral arms wind out from the nucleus ofNGC 4414 Young blue stars throng the armswhile older redder stars populate the nuclearbulge NASATHE HUBBLE HERITAGE TEAM 983080STSCIAURA983081
The ldquoMicerdquo are two spiral galaxies in the process of merging Gravity haspulled material out of each to form long tails while compressed gas cloufuel new star formation NASAESATHE ACS SCIENCE TEAM
M87
A high-speed jet shoots from the heart of the giant
elliptical galaxy M87 (upper left) in the Virgo cluster A black hole of some3 billion solar masses drives this activity NASATHE HUBBLE HERITAGE TEAM 983080STSCIAUR
Giant elliptical galaxies M84 (left) and M86 (right)each contain a trillion stars These two dominate the central regionof the nearby Virgo cluster a collection of some 2000 galaxies NOAOAURA
M84 amp M86
10 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1114
NGC 68221760 kly
Ursa Minor Dwarf 225 kly
Draco Dwarf
248 kly MilkyWay
SMC189 kly
NGC 1852020 kly
NGC 1472460 kly
Andromeda Galax
2510 kly
Pinwheel Galaxy (M33)
2770 kly
Leo I818 kly
Leo II750 kly
Sextans
Dwarf 293 kly
LMC160 kly
The Local GroupThe Milky Way and Andromeda galaxiesrule the Local Group accounting for morethan half its mass Distances from our galaxy
are given in thousands of light-years (kly) ASTRONOMY ROEN KELLY
F U N
If you examine the brightestgalaxies some 75 percent arespirals 20 percent ellipticalsand 5 percent irregularsIncluding faint dwarfs skews
the numbers to 30 percentspirals 60 percent ellipticalsand 10 percent irregulars
F A C T
ndromeda Galaxyghtly bigger than the Milky Way the Andromeda Galaxy (M31) containsme 500 billion stars Located 25 million light-years away the galaxy can bempsed with the naked eye MICHAEL STECKER
he LMC
e Large Magellanicoud (LMC) is an irregular
axy about 160000ht-years from Earth Thedish cloud at top righthe Tarantula Nebula
e largest known regionstar formation LUKE DODD
F U N
The Large and SmallMagellanic Clouds (LMCand SMC) are satellite galax-ies to the Milky Way They liedeep in the southern sky andwere not seen by Europeansuntil Magellanrsquos around-the-world voyage
F A C T
F U N
The Andromeda Galaxy mayseem a good neighbor tothe Milky Way but it wonrsquotalways be so Astronomersthink that in approximately 5billion years our two galaxieswill collide and merge
F A C T
8112019 Your Guide to Planets Stars and Galaxies
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H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
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Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
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Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
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Moon Planet Distance from Orbital period Diameter Den
planet (miles) (days) (miles) (wat
Moon Earth 238900 2732 2160 3
Io Jupiter 262100 177 2263 3Europa Jupiter 417000 355 1940 3
Ganymede Jupiter 665100 715 3271 1
Callisto Jupiter 1169900 1669 2994 1
Enceladus Saturn 147900 137 313 1
Tethys Saturn 183100 189 660 1
Dione Saturn 234500 274 698 1
Rhea Saturn 327500 452 949 1
Titan Saturn 759200 1595 3200 1
Iapetus Saturn 2212600 7933 913 1
Ariel Uranus 118600 252 719 1
Umbriel Uranus 165300 414 727 1
Titania Uranus 271100 871 980 1
Oberon Uranus 362600 1346 946 1
Triton Neptune 220400 588 1681 2Charon Pluto 12200 639 753 1
Major moons
ErosHale-Bopp
Callisto
One of the brightest comets of the past40 years Hale-Bopp wowed observersin 1997 It had a nucleus 25 miles wideand a tail that stretched more than 100million miles BILL AND SALLY FLETCHER
Saturnrsquos icy moon Enceladus reflectsmore than 90 percent of the sunlightthat reaches it the highest percentageof any object in the solar system NASAJPL
PhobosLike most small moons in the solarsystem Marsrsquo Phobos measures just a
few miles across and has an irregularshape NASAJPLMSSS
F U N
Iorsquos active volcanoes andEuroparsquos underground oceanof liquid water both stemfrom enormous tidal forcesmdash which flex and heat themoonsrsquo interiors mdash exerted
by Jupiterrsquos massive gravityF A C T
Multi-ringed impact basins some stretching more than1000 miles formed on Jupiterrsquos Callisto when massiveimpacts left concentric fractures and faults NASAJPL
Potato-shaped asteroid Eros some 20 miles long looks likea lot of other modest-sized asteroids but this object mightone day wander dangerously close to Earth NASAJHUAPL
Triton
nceladus
eyser-like plumes eposited the darktreaks seen on Nep-unersquos moon Triton At aemperature of ndash390deg F Tritonas the coldest surface known in
he solar system NASAJPL
F U N
Saturnrsquos enigmatic outermoon Iapetus has a splitpersonality Half of its surface
appears as dark as freshly laidasphalt while the oppositehemisphere reflects as muchlight as newly fallen snow
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 614
Protostar
A
ll stars begin their lives in the vast clouds of gas and
dust that litter galaxies like the Milky Way A single
cloud can produce hundreds or even thousands of
stars Something triggers the cloud to start collaps-
ing mdash perhaps strong winds from a massive star ora nearby supernova explosion mdash and gravity works its magic
The cloud fragments and each pocket of material continues to
contract and heat up
The contracting star becomes stable when it starts to gener-
ate energy by nuclear fusion Four hydrogen atoms combine to
form one helium atom Because one helium weighs slightly less
than the four hydrogens combined the reaction creates energy
according to Einsteinrsquos equation E=mc2
The biggest stars contain up to about 120 times as much
material as the Sun They burn hot and use their fuel rapidly
These luminaries may have a surface temperature of 70000deg F
radiate nearly a million times the Sunrsquos light and survive onlya few million years
The Sun shines at about 10000deg F and will last some 10
billion years (itrsquos about halfway through now) The smallest
stars have 8 percent of the Sunrsquos mass and glow at only 3000deg
to 4000deg F mdash so dim that they can shine for a trillion years
Once a star exhausts its hydrogen fuel the end is nigh First
it swells into a red giant expanding to a diameter of hundreds of
millions of miles and cooling to a few thousand degrees It may
tap into more nuclear reactions converting helium to carbon
for example but eventually those
fuels run out as well Stars with up
to about eight times the Sunrsquos mass
eventually puff off their outer layers
and form glowing gas clouds known
as planetary nebulae The star itself
settles down as a white dwarf
More massive stars typically die
in supernova explosions Such explo-
sions scatter the heavy elements built
up during the starrsquos life forming
the raw material for new stars and
perhaps planets The collapsed remnant of the exploded star
becomes either a rapidly spinning neutron star or a black hole
whose gravity is so strong that not even light can escape
Stars inour galaxy
The Sun
Most people think of the Sun as the anchor of our solar system mdash andthatrsquos certainly true It contains 998 percent of all the matter in the solasystem But to astronomers the Sun has even more importance It is theonly star in the universe that appears as more than a point of lightthrough a telescope Detailed observations of the Sun led scientists tounderstand how stars shine how they radiate energy and even howhuge storms wrack their surfaces NASASOHO
The life of a Sun-like star
N49
Heavy elements forged in a massive star spreadout at thousands of milesper second in supernovaremnant N49 One daythese elements may beincluded in a new stellargeneration NASATHE HUBBLE
HERITAGE TEAM 983080STSCIAURA983081F U N
Astronomers divide stars intoseven main spectral classesGenerations of students havelearned the sequence byusing the first letters in thesentence ldquoOh Be A Fine Girl(or Guy) Kiss Merdquo
F A C T
6 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 714
Main sequencesolar-type star
Red giant Asymptotic-giant-branch star Protoplanetary nebula Planetary nebula White dw
Spectral Mass Temperature Main sequence Examples class (Sun=1) (Fahrenheit) radius (Sun=1)
O 20ndash120 greater than 55000deg 12ndash25 Zeta (ζ) Puppis
B 4ndash20 17100degndash55000deg 4ndash12 Rigel Spica
A 2ndash4 12300degndash17100deg 15ndash4 Sirius Vega
F 105ndash2 10300degndash12300deg 11ndash15 Canopus Procyon
G 08ndash105 9000degndash10300deg 085ndash11 Sun Capella
K 05ndash08 6700degndash9000deg 06ndash085 Aldebaran Arcturu
M 008ndash05 3100degndash6700deg 01ndash06 Antares Betelgeus
Star characteristics
Stars like the Sun condense out of a gaseous cloud The growing protostar develops a disk (which may form planets)and shoots out material before settling down as a main sequence star converting hydrogen to helium Once the hydro-gen runs out the star swells to a red giant and becomes unstable as an asymptotic-giant-branch star before puffing offits outer layers as a planetary nebula The starrsquos core remains as a dense white dwarf ASTRONOMY ROEN KELLY
Surface of the SunCatrsquos Eye Nebula
Cone Nebula
New stars form from clouds of gas and dust such asthe Cone Nebula Hot stars ionize the surroundinghydrogen gas which glows with a characteristic redcolor NASAESATHE ACS SCIENCE TEAM
When the Sun dies in 5 billion years it may resem-ble the symmetric Catrsquos Eye Nebula Here glowingstrands of ionized gas mark where a dying starrepeatedly shed its outer layers NASAESAHEIC
THE HUBBLE HERI TAGE TEAM 983080STSCIAURA983081
An intricate honeycomb on the Sunrsquos surface marks regionswhere heat (bright areas) rises and cooler material (dark areasinks in a process called convection ROYAL SWEDISH ACADEMY OF SCIE
F U N
To shine as brightly as it doesand nourish life on Earth theSun must convert 600 milliontons of hydrogen into heliumevery second
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 814
S c u t
u m - C
e n t a
u r u s
A r m
Norma Ar m
C e n t r a l b a r
S a g i t t a r i u s A
r m
P e r s e
u s A
r m
O r i o n
S p u r
Sun
The MilkyWay Galaxy
Head outside on a clear dark summerrsquos night and your
eyes will be greeted by thousands of stars All of them
belong to our galaxy as does virtually everything
else you can see with the naked eye If you let youreyes adjust to the darkness yoursquoll see a gauzy whitish
band running across the sky This is the Milky Way mdash the com-
bined light of countless stars mdash and the feature that lends its
name to our galaxy
The Milky Way is a giant barred spiral galaxy that stretches
about 120000 light-years from end to end but whose disk mea-
sures only some 1000 light-years thick The central bar extends
28000 light-years The Sun lies about halfway between the gal-
axyrsquos center and edge and revolves at approximately 150 miles
per second taking roughly 225 mil-
lion years to complete one circuit of
the galactic hubThe most obvious sights of the
galaxy are stars Astronomers esti-
mate between 200 and 400 billion
populate the Milky Way Galaxy
(most are hidden from view or
extremely faint so a precise count
isnrsquot possible)
Because the hottest brightest stars
are also short-lived mdash and the spiral
arms are the only place in the galaxy
with active star formation mdash the arms stand out The clouds
of gas and dust from which stars form also call the spiral arms
home as do the open star clusters that emerge from them
The nuclear bulge of the galaxy consists mostly of old stars
It measures about 12000 light-years across At the galaxyrsquos heart
lies a supermassive black hole that weighs approximately 4 mil-
lion Suns Surrounding the bulge and disk is a vast spherical
halo that stretches some 300000 light-years
The most prominent members of the halo are globular
clusters These ancient collections of up to a million stars each
were born at the same time as the galaxy some 12 billion to 13
billion years ago They contain few heavy elements because they
formed before supernova explosions had enriched the inter-
stellar medium with them
Structure of the Milky Way
The Pleiades M3
Star formation in Cygnus
F U N
Most naked-eye stars aremassive and highly luminousones that shine across greatdistances But this gives a dis-torted view of the galaxy as awhole In actuality cool dimM-type stars make up abouttwo-thirds of all stars in theMilky Way
F A C T
A stellar nursery in Cygnus harbors many massive young stars Invisibloptical light the DR21 complex shows up when viewed in dust-penetratinfrared radiation NASAJPL983085CALTECHA MARSTON 983080ESTECESA983081
Open star clusters like the Pleiadescontain dozens to hundreds of starsThese groups lie in our galaxyrsquos spi-ral arms and disperse over billionsof years JASON WARE
Globular star clusters have eas long as the Milky Way M3 p500000 stars in a sphere 160 years across S KAFKA AND K HONEYC
INDIANA UNIVERSITYWIYNNOAONSF
The Sun lies in the Orion Spur one of several arms and smaller appenda
where our galaxy creates stars Astronomers name the spiral arms after tconstellation where they appear prominent NASAJPL983085CALTECHR HURT 983080SSC983085CALT8 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 914
F U N
Much of the Milky Way Galaxyand its structure remain hid-den to earthbound observersbecause dust chokes the spi-ral arms Itrsquos like being in thewoods and trying to discernthe forestrsquos form
F A C T
F U N
In measuring distances inthe galaxy and the universeastronomers use a unitknown as the l ight-year Itrepresents the distance abeam of light travels in oneyear At 186000 miles persecond light traverses 59trillion miles in a year
F A C T
band of dust cuts through the Milky Way blocking light
om distant stars If not for all the dust the galaxyrsquos centerould shine brighter than the brightest star STEVE THORNTON
F U N
A century ago astronomersthought the Sun occupiedthe center of the galaxy Butcareful studies of globularclusters which orbit theMilky Wayrsquos center and tendto gather in the constellationSagittarius show we livehalfway to the edge
F A C T
F U N
How do astronomers knowa black hole resides at theMilky Wayrsquos center They havefound stars near the centralhub orbiting so fast that theymust be circling an invisibleobject containing 4 millionsolar masses
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1014
T
he collections of stars gas and dust known as galaxies
form the building blocks of the observable universe
Roughly 125 billion galaxies populate the cosmos and
they come in all shapes and sizes Astronomers divide
galaxies into three major categories spirals ellipticalsand irregulars A spiral has a broad disk containing clouds of gas
and dust and from two to several spiral arms a nuclear bulge of
old stars and a spherical halo that envelops both
Approximately one-third of spirals exhibit central bars mdash a
symmetric concentration of stars and sometimes gas and dust
that crosses the nucleus and connects with the outer spiral arms
(Recent studies show the Milky Way possesses a significant bar)
The diameters of spirals range from roughly 20000 to more
than 100000 light-years and they contain anywhere from sev-
eral billion to several hundred billion stars
Elliptical galaxies appear spherical or flattened in shape
They possess little of the gas and dust seen in the disks of spiralgalaxies so they donrsquot generate any new stars Ellipticals show
the widest range in size of any galaxy type Giant ellipticals can
span 1 million light-years and contain several trillion stars
dwarf ellipticals may be only a few thousand light-years across
and have millions of stars An important intermediate type of
galaxy has a disk like a spiral galaxy but contains no gas or dust
Astronomers call this type of galaxy a lenticular
Irregular galaxies donrsquot show any symmetry or organized
spiral structure The category exists basically as a catchall for
galaxies that donrsquot fit either the spiral or elliptical classification
Irregulars can be big containing up to 100 billion stars or as
small as dwarf ellipticals Astronomers think most irregulars
result from the collisions or mergers of two or more galaxies
The gravitational interactions disrupt normal spiral or elliptical
structure leaving behind a chaotic appearance
Most galaxies belong to groups with dozens of members or
to clusters with up to thousands of members The Milky Way
joins with the slightly larger Andromeda Galaxy to form the
cornerstones of the Local Group a collection of roughly 50 gal-
axies that spans several million light-years The vast majority of
Local Group galaxies are dwarf ellipticals and irregulars Small
groups generally have a few dozen member galaxies but clusters
can contain several thousand galaxies The Virgo cluster located
50 million light-years away is the nearest large cluster to Earth
Galaxies
in theuniverse
The Mice
NGC 4414
Multiple spiral arms wind out from the nucleus ofNGC 4414 Young blue stars throng the armswhile older redder stars populate the nuclearbulge NASATHE HUBBLE HERITAGE TEAM 983080STSCIAURA983081
The ldquoMicerdquo are two spiral galaxies in the process of merging Gravity haspulled material out of each to form long tails while compressed gas cloufuel new star formation NASAESATHE ACS SCIENCE TEAM
M87
A high-speed jet shoots from the heart of the giant
elliptical galaxy M87 (upper left) in the Virgo cluster A black hole of some3 billion solar masses drives this activity NASATHE HUBBLE HERITAGE TEAM 983080STSCIAUR
Giant elliptical galaxies M84 (left) and M86 (right)each contain a trillion stars These two dominate the central regionof the nearby Virgo cluster a collection of some 2000 galaxies NOAOAURA
M84 amp M86
10 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1114
NGC 68221760 kly
Ursa Minor Dwarf 225 kly
Draco Dwarf
248 kly MilkyWay
SMC189 kly
NGC 1852020 kly
NGC 1472460 kly
Andromeda Galax
2510 kly
Pinwheel Galaxy (M33)
2770 kly
Leo I818 kly
Leo II750 kly
Sextans
Dwarf 293 kly
LMC160 kly
The Local GroupThe Milky Way and Andromeda galaxiesrule the Local Group accounting for morethan half its mass Distances from our galaxy
are given in thousands of light-years (kly) ASTRONOMY ROEN KELLY
F U N
If you examine the brightestgalaxies some 75 percent arespirals 20 percent ellipticalsand 5 percent irregularsIncluding faint dwarfs skews
the numbers to 30 percentspirals 60 percent ellipticalsand 10 percent irregulars
F A C T
ndromeda Galaxyghtly bigger than the Milky Way the Andromeda Galaxy (M31) containsme 500 billion stars Located 25 million light-years away the galaxy can bempsed with the naked eye MICHAEL STECKER
he LMC
e Large Magellanicoud (LMC) is an irregular
axy about 160000ht-years from Earth Thedish cloud at top righthe Tarantula Nebula
e largest known regionstar formation LUKE DODD
F U N
The Large and SmallMagellanic Clouds (LMCand SMC) are satellite galax-ies to the Milky Way They liedeep in the southern sky andwere not seen by Europeansuntil Magellanrsquos around-the-world voyage
F A C T
F U N
The Andromeda Galaxy mayseem a good neighbor tothe Milky Way but it wonrsquotalways be so Astronomersthink that in approximately 5billion years our two galaxieswill collide and merge
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1214
H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1314
Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1414
Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 614
Protostar
A
ll stars begin their lives in the vast clouds of gas and
dust that litter galaxies like the Milky Way A single
cloud can produce hundreds or even thousands of
stars Something triggers the cloud to start collaps-
ing mdash perhaps strong winds from a massive star ora nearby supernova explosion mdash and gravity works its magic
The cloud fragments and each pocket of material continues to
contract and heat up
The contracting star becomes stable when it starts to gener-
ate energy by nuclear fusion Four hydrogen atoms combine to
form one helium atom Because one helium weighs slightly less
than the four hydrogens combined the reaction creates energy
according to Einsteinrsquos equation E=mc2
The biggest stars contain up to about 120 times as much
material as the Sun They burn hot and use their fuel rapidly
These luminaries may have a surface temperature of 70000deg F
radiate nearly a million times the Sunrsquos light and survive onlya few million years
The Sun shines at about 10000deg F and will last some 10
billion years (itrsquos about halfway through now) The smallest
stars have 8 percent of the Sunrsquos mass and glow at only 3000deg
to 4000deg F mdash so dim that they can shine for a trillion years
Once a star exhausts its hydrogen fuel the end is nigh First
it swells into a red giant expanding to a diameter of hundreds of
millions of miles and cooling to a few thousand degrees It may
tap into more nuclear reactions converting helium to carbon
for example but eventually those
fuels run out as well Stars with up
to about eight times the Sunrsquos mass
eventually puff off their outer layers
and form glowing gas clouds known
as planetary nebulae The star itself
settles down as a white dwarf
More massive stars typically die
in supernova explosions Such explo-
sions scatter the heavy elements built
up during the starrsquos life forming
the raw material for new stars and
perhaps planets The collapsed remnant of the exploded star
becomes either a rapidly spinning neutron star or a black hole
whose gravity is so strong that not even light can escape
Stars inour galaxy
The Sun
Most people think of the Sun as the anchor of our solar system mdash andthatrsquos certainly true It contains 998 percent of all the matter in the solasystem But to astronomers the Sun has even more importance It is theonly star in the universe that appears as more than a point of lightthrough a telescope Detailed observations of the Sun led scientists tounderstand how stars shine how they radiate energy and even howhuge storms wrack their surfaces NASASOHO
The life of a Sun-like star
N49
Heavy elements forged in a massive star spreadout at thousands of milesper second in supernovaremnant N49 One daythese elements may beincluded in a new stellargeneration NASATHE HUBBLE
HERITAGE TEAM 983080STSCIAURA983081F U N
Astronomers divide stars intoseven main spectral classesGenerations of students havelearned the sequence byusing the first letters in thesentence ldquoOh Be A Fine Girl(or Guy) Kiss Merdquo
F A C T
6 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 714
Main sequencesolar-type star
Red giant Asymptotic-giant-branch star Protoplanetary nebula Planetary nebula White dw
Spectral Mass Temperature Main sequence Examples class (Sun=1) (Fahrenheit) radius (Sun=1)
O 20ndash120 greater than 55000deg 12ndash25 Zeta (ζ) Puppis
B 4ndash20 17100degndash55000deg 4ndash12 Rigel Spica
A 2ndash4 12300degndash17100deg 15ndash4 Sirius Vega
F 105ndash2 10300degndash12300deg 11ndash15 Canopus Procyon
G 08ndash105 9000degndash10300deg 085ndash11 Sun Capella
K 05ndash08 6700degndash9000deg 06ndash085 Aldebaran Arcturu
M 008ndash05 3100degndash6700deg 01ndash06 Antares Betelgeus
Star characteristics
Stars like the Sun condense out of a gaseous cloud The growing protostar develops a disk (which may form planets)and shoots out material before settling down as a main sequence star converting hydrogen to helium Once the hydro-gen runs out the star swells to a red giant and becomes unstable as an asymptotic-giant-branch star before puffing offits outer layers as a planetary nebula The starrsquos core remains as a dense white dwarf ASTRONOMY ROEN KELLY
Surface of the SunCatrsquos Eye Nebula
Cone Nebula
New stars form from clouds of gas and dust such asthe Cone Nebula Hot stars ionize the surroundinghydrogen gas which glows with a characteristic redcolor NASAESATHE ACS SCIENCE TEAM
When the Sun dies in 5 billion years it may resem-ble the symmetric Catrsquos Eye Nebula Here glowingstrands of ionized gas mark where a dying starrepeatedly shed its outer layers NASAESAHEIC
THE HUBBLE HERI TAGE TEAM 983080STSCIAURA983081
An intricate honeycomb on the Sunrsquos surface marks regionswhere heat (bright areas) rises and cooler material (dark areasinks in a process called convection ROYAL SWEDISH ACADEMY OF SCIE
F U N
To shine as brightly as it doesand nourish life on Earth theSun must convert 600 milliontons of hydrogen into heliumevery second
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 814
S c u t
u m - C
e n t a
u r u s
A r m
Norma Ar m
C e n t r a l b a r
S a g i t t a r i u s A
r m
P e r s e
u s A
r m
O r i o n
S p u r
Sun
The MilkyWay Galaxy
Head outside on a clear dark summerrsquos night and your
eyes will be greeted by thousands of stars All of them
belong to our galaxy as does virtually everything
else you can see with the naked eye If you let youreyes adjust to the darkness yoursquoll see a gauzy whitish
band running across the sky This is the Milky Way mdash the com-
bined light of countless stars mdash and the feature that lends its
name to our galaxy
The Milky Way is a giant barred spiral galaxy that stretches
about 120000 light-years from end to end but whose disk mea-
sures only some 1000 light-years thick The central bar extends
28000 light-years The Sun lies about halfway between the gal-
axyrsquos center and edge and revolves at approximately 150 miles
per second taking roughly 225 mil-
lion years to complete one circuit of
the galactic hubThe most obvious sights of the
galaxy are stars Astronomers esti-
mate between 200 and 400 billion
populate the Milky Way Galaxy
(most are hidden from view or
extremely faint so a precise count
isnrsquot possible)
Because the hottest brightest stars
are also short-lived mdash and the spiral
arms are the only place in the galaxy
with active star formation mdash the arms stand out The clouds
of gas and dust from which stars form also call the spiral arms
home as do the open star clusters that emerge from them
The nuclear bulge of the galaxy consists mostly of old stars
It measures about 12000 light-years across At the galaxyrsquos heart
lies a supermassive black hole that weighs approximately 4 mil-
lion Suns Surrounding the bulge and disk is a vast spherical
halo that stretches some 300000 light-years
The most prominent members of the halo are globular
clusters These ancient collections of up to a million stars each
were born at the same time as the galaxy some 12 billion to 13
billion years ago They contain few heavy elements because they
formed before supernova explosions had enriched the inter-
stellar medium with them
Structure of the Milky Way
The Pleiades M3
Star formation in Cygnus
F U N
Most naked-eye stars aremassive and highly luminousones that shine across greatdistances But this gives a dis-torted view of the galaxy as awhole In actuality cool dimM-type stars make up abouttwo-thirds of all stars in theMilky Way
F A C T
A stellar nursery in Cygnus harbors many massive young stars Invisibloptical light the DR21 complex shows up when viewed in dust-penetratinfrared radiation NASAJPL983085CALTECHA MARSTON 983080ESTECESA983081
Open star clusters like the Pleiadescontain dozens to hundreds of starsThese groups lie in our galaxyrsquos spi-ral arms and disperse over billionsof years JASON WARE
Globular star clusters have eas long as the Milky Way M3 p500000 stars in a sphere 160 years across S KAFKA AND K HONEYC
INDIANA UNIVERSITYWIYNNOAONSF
The Sun lies in the Orion Spur one of several arms and smaller appenda
where our galaxy creates stars Astronomers name the spiral arms after tconstellation where they appear prominent NASAJPL983085CALTECHR HURT 983080SSC983085CALT8 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 914
F U N
Much of the Milky Way Galaxyand its structure remain hid-den to earthbound observersbecause dust chokes the spi-ral arms Itrsquos like being in thewoods and trying to discernthe forestrsquos form
F A C T
F U N
In measuring distances inthe galaxy and the universeastronomers use a unitknown as the l ight-year Itrepresents the distance abeam of light travels in oneyear At 186000 miles persecond light traverses 59trillion miles in a year
F A C T
band of dust cuts through the Milky Way blocking light
om distant stars If not for all the dust the galaxyrsquos centerould shine brighter than the brightest star STEVE THORNTON
F U N
A century ago astronomersthought the Sun occupiedthe center of the galaxy Butcareful studies of globularclusters which orbit theMilky Wayrsquos center and tendto gather in the constellationSagittarius show we livehalfway to the edge
F A C T
F U N
How do astronomers knowa black hole resides at theMilky Wayrsquos center They havefound stars near the centralhub orbiting so fast that theymust be circling an invisibleobject containing 4 millionsolar masses
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1014
T
he collections of stars gas and dust known as galaxies
form the building blocks of the observable universe
Roughly 125 billion galaxies populate the cosmos and
they come in all shapes and sizes Astronomers divide
galaxies into three major categories spirals ellipticalsand irregulars A spiral has a broad disk containing clouds of gas
and dust and from two to several spiral arms a nuclear bulge of
old stars and a spherical halo that envelops both
Approximately one-third of spirals exhibit central bars mdash a
symmetric concentration of stars and sometimes gas and dust
that crosses the nucleus and connects with the outer spiral arms
(Recent studies show the Milky Way possesses a significant bar)
The diameters of spirals range from roughly 20000 to more
than 100000 light-years and they contain anywhere from sev-
eral billion to several hundred billion stars
Elliptical galaxies appear spherical or flattened in shape
They possess little of the gas and dust seen in the disks of spiralgalaxies so they donrsquot generate any new stars Ellipticals show
the widest range in size of any galaxy type Giant ellipticals can
span 1 million light-years and contain several trillion stars
dwarf ellipticals may be only a few thousand light-years across
and have millions of stars An important intermediate type of
galaxy has a disk like a spiral galaxy but contains no gas or dust
Astronomers call this type of galaxy a lenticular
Irregular galaxies donrsquot show any symmetry or organized
spiral structure The category exists basically as a catchall for
galaxies that donrsquot fit either the spiral or elliptical classification
Irregulars can be big containing up to 100 billion stars or as
small as dwarf ellipticals Astronomers think most irregulars
result from the collisions or mergers of two or more galaxies
The gravitational interactions disrupt normal spiral or elliptical
structure leaving behind a chaotic appearance
Most galaxies belong to groups with dozens of members or
to clusters with up to thousands of members The Milky Way
joins with the slightly larger Andromeda Galaxy to form the
cornerstones of the Local Group a collection of roughly 50 gal-
axies that spans several million light-years The vast majority of
Local Group galaxies are dwarf ellipticals and irregulars Small
groups generally have a few dozen member galaxies but clusters
can contain several thousand galaxies The Virgo cluster located
50 million light-years away is the nearest large cluster to Earth
Galaxies
in theuniverse
The Mice
NGC 4414
Multiple spiral arms wind out from the nucleus ofNGC 4414 Young blue stars throng the armswhile older redder stars populate the nuclearbulge NASATHE HUBBLE HERITAGE TEAM 983080STSCIAURA983081
The ldquoMicerdquo are two spiral galaxies in the process of merging Gravity haspulled material out of each to form long tails while compressed gas cloufuel new star formation NASAESATHE ACS SCIENCE TEAM
M87
A high-speed jet shoots from the heart of the giant
elliptical galaxy M87 (upper left) in the Virgo cluster A black hole of some3 billion solar masses drives this activity NASATHE HUBBLE HERITAGE TEAM 983080STSCIAUR
Giant elliptical galaxies M84 (left) and M86 (right)each contain a trillion stars These two dominate the central regionof the nearby Virgo cluster a collection of some 2000 galaxies NOAOAURA
M84 amp M86
10 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1114
NGC 68221760 kly
Ursa Minor Dwarf 225 kly
Draco Dwarf
248 kly MilkyWay
SMC189 kly
NGC 1852020 kly
NGC 1472460 kly
Andromeda Galax
2510 kly
Pinwheel Galaxy (M33)
2770 kly
Leo I818 kly
Leo II750 kly
Sextans
Dwarf 293 kly
LMC160 kly
The Local GroupThe Milky Way and Andromeda galaxiesrule the Local Group accounting for morethan half its mass Distances from our galaxy
are given in thousands of light-years (kly) ASTRONOMY ROEN KELLY
F U N
If you examine the brightestgalaxies some 75 percent arespirals 20 percent ellipticalsand 5 percent irregularsIncluding faint dwarfs skews
the numbers to 30 percentspirals 60 percent ellipticalsand 10 percent irregulars
F A C T
ndromeda Galaxyghtly bigger than the Milky Way the Andromeda Galaxy (M31) containsme 500 billion stars Located 25 million light-years away the galaxy can bempsed with the naked eye MICHAEL STECKER
he LMC
e Large Magellanicoud (LMC) is an irregular
axy about 160000ht-years from Earth Thedish cloud at top righthe Tarantula Nebula
e largest known regionstar formation LUKE DODD
F U N
The Large and SmallMagellanic Clouds (LMCand SMC) are satellite galax-ies to the Milky Way They liedeep in the southern sky andwere not seen by Europeansuntil Magellanrsquos around-the-world voyage
F A C T
F U N
The Andromeda Galaxy mayseem a good neighbor tothe Milky Way but it wonrsquotalways be so Astronomersthink that in approximately 5billion years our two galaxieswill collide and merge
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1214
H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1314
Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1414
Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 714
Main sequencesolar-type star
Red giant Asymptotic-giant-branch star Protoplanetary nebula Planetary nebula White dw
Spectral Mass Temperature Main sequence Examples class (Sun=1) (Fahrenheit) radius (Sun=1)
O 20ndash120 greater than 55000deg 12ndash25 Zeta (ζ) Puppis
B 4ndash20 17100degndash55000deg 4ndash12 Rigel Spica
A 2ndash4 12300degndash17100deg 15ndash4 Sirius Vega
F 105ndash2 10300degndash12300deg 11ndash15 Canopus Procyon
G 08ndash105 9000degndash10300deg 085ndash11 Sun Capella
K 05ndash08 6700degndash9000deg 06ndash085 Aldebaran Arcturu
M 008ndash05 3100degndash6700deg 01ndash06 Antares Betelgeus
Star characteristics
Stars like the Sun condense out of a gaseous cloud The growing protostar develops a disk (which may form planets)and shoots out material before settling down as a main sequence star converting hydrogen to helium Once the hydro-gen runs out the star swells to a red giant and becomes unstable as an asymptotic-giant-branch star before puffing offits outer layers as a planetary nebula The starrsquos core remains as a dense white dwarf ASTRONOMY ROEN KELLY
Surface of the SunCatrsquos Eye Nebula
Cone Nebula
New stars form from clouds of gas and dust such asthe Cone Nebula Hot stars ionize the surroundinghydrogen gas which glows with a characteristic redcolor NASAESATHE ACS SCIENCE TEAM
When the Sun dies in 5 billion years it may resem-ble the symmetric Catrsquos Eye Nebula Here glowingstrands of ionized gas mark where a dying starrepeatedly shed its outer layers NASAESAHEIC
THE HUBBLE HERI TAGE TEAM 983080STSCIAURA983081
An intricate honeycomb on the Sunrsquos surface marks regionswhere heat (bright areas) rises and cooler material (dark areasinks in a process called convection ROYAL SWEDISH ACADEMY OF SCIE
F U N
To shine as brightly as it doesand nourish life on Earth theSun must convert 600 milliontons of hydrogen into heliumevery second
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 814
S c u t
u m - C
e n t a
u r u s
A r m
Norma Ar m
C e n t r a l b a r
S a g i t t a r i u s A
r m
P e r s e
u s A
r m
O r i o n
S p u r
Sun
The MilkyWay Galaxy
Head outside on a clear dark summerrsquos night and your
eyes will be greeted by thousands of stars All of them
belong to our galaxy as does virtually everything
else you can see with the naked eye If you let youreyes adjust to the darkness yoursquoll see a gauzy whitish
band running across the sky This is the Milky Way mdash the com-
bined light of countless stars mdash and the feature that lends its
name to our galaxy
The Milky Way is a giant barred spiral galaxy that stretches
about 120000 light-years from end to end but whose disk mea-
sures only some 1000 light-years thick The central bar extends
28000 light-years The Sun lies about halfway between the gal-
axyrsquos center and edge and revolves at approximately 150 miles
per second taking roughly 225 mil-
lion years to complete one circuit of
the galactic hubThe most obvious sights of the
galaxy are stars Astronomers esti-
mate between 200 and 400 billion
populate the Milky Way Galaxy
(most are hidden from view or
extremely faint so a precise count
isnrsquot possible)
Because the hottest brightest stars
are also short-lived mdash and the spiral
arms are the only place in the galaxy
with active star formation mdash the arms stand out The clouds
of gas and dust from which stars form also call the spiral arms
home as do the open star clusters that emerge from them
The nuclear bulge of the galaxy consists mostly of old stars
It measures about 12000 light-years across At the galaxyrsquos heart
lies a supermassive black hole that weighs approximately 4 mil-
lion Suns Surrounding the bulge and disk is a vast spherical
halo that stretches some 300000 light-years
The most prominent members of the halo are globular
clusters These ancient collections of up to a million stars each
were born at the same time as the galaxy some 12 billion to 13
billion years ago They contain few heavy elements because they
formed before supernova explosions had enriched the inter-
stellar medium with them
Structure of the Milky Way
The Pleiades M3
Star formation in Cygnus
F U N
Most naked-eye stars aremassive and highly luminousones that shine across greatdistances But this gives a dis-torted view of the galaxy as awhole In actuality cool dimM-type stars make up abouttwo-thirds of all stars in theMilky Way
F A C T
A stellar nursery in Cygnus harbors many massive young stars Invisibloptical light the DR21 complex shows up when viewed in dust-penetratinfrared radiation NASAJPL983085CALTECHA MARSTON 983080ESTECESA983081
Open star clusters like the Pleiadescontain dozens to hundreds of starsThese groups lie in our galaxyrsquos spi-ral arms and disperse over billionsof years JASON WARE
Globular star clusters have eas long as the Milky Way M3 p500000 stars in a sphere 160 years across S KAFKA AND K HONEYC
INDIANA UNIVERSITYWIYNNOAONSF
The Sun lies in the Orion Spur one of several arms and smaller appenda
where our galaxy creates stars Astronomers name the spiral arms after tconstellation where they appear prominent NASAJPL983085CALTECHR HURT 983080SSC983085CALT8 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 914
F U N
Much of the Milky Way Galaxyand its structure remain hid-den to earthbound observersbecause dust chokes the spi-ral arms Itrsquos like being in thewoods and trying to discernthe forestrsquos form
F A C T
F U N
In measuring distances inthe galaxy and the universeastronomers use a unitknown as the l ight-year Itrepresents the distance abeam of light travels in oneyear At 186000 miles persecond light traverses 59trillion miles in a year
F A C T
band of dust cuts through the Milky Way blocking light
om distant stars If not for all the dust the galaxyrsquos centerould shine brighter than the brightest star STEVE THORNTON
F U N
A century ago astronomersthought the Sun occupiedthe center of the galaxy Butcareful studies of globularclusters which orbit theMilky Wayrsquos center and tendto gather in the constellationSagittarius show we livehalfway to the edge
F A C T
F U N
How do astronomers knowa black hole resides at theMilky Wayrsquos center They havefound stars near the centralhub orbiting so fast that theymust be circling an invisibleobject containing 4 millionsolar masses
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1014
T
he collections of stars gas and dust known as galaxies
form the building blocks of the observable universe
Roughly 125 billion galaxies populate the cosmos and
they come in all shapes and sizes Astronomers divide
galaxies into three major categories spirals ellipticalsand irregulars A spiral has a broad disk containing clouds of gas
and dust and from two to several spiral arms a nuclear bulge of
old stars and a spherical halo that envelops both
Approximately one-third of spirals exhibit central bars mdash a
symmetric concentration of stars and sometimes gas and dust
that crosses the nucleus and connects with the outer spiral arms
(Recent studies show the Milky Way possesses a significant bar)
The diameters of spirals range from roughly 20000 to more
than 100000 light-years and they contain anywhere from sev-
eral billion to several hundred billion stars
Elliptical galaxies appear spherical or flattened in shape
They possess little of the gas and dust seen in the disks of spiralgalaxies so they donrsquot generate any new stars Ellipticals show
the widest range in size of any galaxy type Giant ellipticals can
span 1 million light-years and contain several trillion stars
dwarf ellipticals may be only a few thousand light-years across
and have millions of stars An important intermediate type of
galaxy has a disk like a spiral galaxy but contains no gas or dust
Astronomers call this type of galaxy a lenticular
Irregular galaxies donrsquot show any symmetry or organized
spiral structure The category exists basically as a catchall for
galaxies that donrsquot fit either the spiral or elliptical classification
Irregulars can be big containing up to 100 billion stars or as
small as dwarf ellipticals Astronomers think most irregulars
result from the collisions or mergers of two or more galaxies
The gravitational interactions disrupt normal spiral or elliptical
structure leaving behind a chaotic appearance
Most galaxies belong to groups with dozens of members or
to clusters with up to thousands of members The Milky Way
joins with the slightly larger Andromeda Galaxy to form the
cornerstones of the Local Group a collection of roughly 50 gal-
axies that spans several million light-years The vast majority of
Local Group galaxies are dwarf ellipticals and irregulars Small
groups generally have a few dozen member galaxies but clusters
can contain several thousand galaxies The Virgo cluster located
50 million light-years away is the nearest large cluster to Earth
Galaxies
in theuniverse
The Mice
NGC 4414
Multiple spiral arms wind out from the nucleus ofNGC 4414 Young blue stars throng the armswhile older redder stars populate the nuclearbulge NASATHE HUBBLE HERITAGE TEAM 983080STSCIAURA983081
The ldquoMicerdquo are two spiral galaxies in the process of merging Gravity haspulled material out of each to form long tails while compressed gas cloufuel new star formation NASAESATHE ACS SCIENCE TEAM
M87
A high-speed jet shoots from the heart of the giant
elliptical galaxy M87 (upper left) in the Virgo cluster A black hole of some3 billion solar masses drives this activity NASATHE HUBBLE HERITAGE TEAM 983080STSCIAUR
Giant elliptical galaxies M84 (left) and M86 (right)each contain a trillion stars These two dominate the central regionof the nearby Virgo cluster a collection of some 2000 galaxies NOAOAURA
M84 amp M86
10 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1114
NGC 68221760 kly
Ursa Minor Dwarf 225 kly
Draco Dwarf
248 kly MilkyWay
SMC189 kly
NGC 1852020 kly
NGC 1472460 kly
Andromeda Galax
2510 kly
Pinwheel Galaxy (M33)
2770 kly
Leo I818 kly
Leo II750 kly
Sextans
Dwarf 293 kly
LMC160 kly
The Local GroupThe Milky Way and Andromeda galaxiesrule the Local Group accounting for morethan half its mass Distances from our galaxy
are given in thousands of light-years (kly) ASTRONOMY ROEN KELLY
F U N
If you examine the brightestgalaxies some 75 percent arespirals 20 percent ellipticalsand 5 percent irregularsIncluding faint dwarfs skews
the numbers to 30 percentspirals 60 percent ellipticalsand 10 percent irregulars
F A C T
ndromeda Galaxyghtly bigger than the Milky Way the Andromeda Galaxy (M31) containsme 500 billion stars Located 25 million light-years away the galaxy can bempsed with the naked eye MICHAEL STECKER
he LMC
e Large Magellanicoud (LMC) is an irregular
axy about 160000ht-years from Earth Thedish cloud at top righthe Tarantula Nebula
e largest known regionstar formation LUKE DODD
F U N
The Large and SmallMagellanic Clouds (LMCand SMC) are satellite galax-ies to the Milky Way They liedeep in the southern sky andwere not seen by Europeansuntil Magellanrsquos around-the-world voyage
F A C T
F U N
The Andromeda Galaxy mayseem a good neighbor tothe Milky Way but it wonrsquotalways be so Astronomersthink that in approximately 5billion years our two galaxieswill collide and merge
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1214
H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1314
Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1414
Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 814
S c u t
u m - C
e n t a
u r u s
A r m
Norma Ar m
C e n t r a l b a r
S a g i t t a r i u s A
r m
P e r s e
u s A
r m
O r i o n
S p u r
Sun
The MilkyWay Galaxy
Head outside on a clear dark summerrsquos night and your
eyes will be greeted by thousands of stars All of them
belong to our galaxy as does virtually everything
else you can see with the naked eye If you let youreyes adjust to the darkness yoursquoll see a gauzy whitish
band running across the sky This is the Milky Way mdash the com-
bined light of countless stars mdash and the feature that lends its
name to our galaxy
The Milky Way is a giant barred spiral galaxy that stretches
about 120000 light-years from end to end but whose disk mea-
sures only some 1000 light-years thick The central bar extends
28000 light-years The Sun lies about halfway between the gal-
axyrsquos center and edge and revolves at approximately 150 miles
per second taking roughly 225 mil-
lion years to complete one circuit of
the galactic hubThe most obvious sights of the
galaxy are stars Astronomers esti-
mate between 200 and 400 billion
populate the Milky Way Galaxy
(most are hidden from view or
extremely faint so a precise count
isnrsquot possible)
Because the hottest brightest stars
are also short-lived mdash and the spiral
arms are the only place in the galaxy
with active star formation mdash the arms stand out The clouds
of gas and dust from which stars form also call the spiral arms
home as do the open star clusters that emerge from them
The nuclear bulge of the galaxy consists mostly of old stars
It measures about 12000 light-years across At the galaxyrsquos heart
lies a supermassive black hole that weighs approximately 4 mil-
lion Suns Surrounding the bulge and disk is a vast spherical
halo that stretches some 300000 light-years
The most prominent members of the halo are globular
clusters These ancient collections of up to a million stars each
were born at the same time as the galaxy some 12 billion to 13
billion years ago They contain few heavy elements because they
formed before supernova explosions had enriched the inter-
stellar medium with them
Structure of the Milky Way
The Pleiades M3
Star formation in Cygnus
F U N
Most naked-eye stars aremassive and highly luminousones that shine across greatdistances But this gives a dis-torted view of the galaxy as awhole In actuality cool dimM-type stars make up abouttwo-thirds of all stars in theMilky Way
F A C T
A stellar nursery in Cygnus harbors many massive young stars Invisibloptical light the DR21 complex shows up when viewed in dust-penetratinfrared radiation NASAJPL983085CALTECHA MARSTON 983080ESTECESA983081
Open star clusters like the Pleiadescontain dozens to hundreds of starsThese groups lie in our galaxyrsquos spi-ral arms and disperse over billionsof years JASON WARE
Globular star clusters have eas long as the Milky Way M3 p500000 stars in a sphere 160 years across S KAFKA AND K HONEYC
INDIANA UNIVERSITYWIYNNOAONSF
The Sun lies in the Orion Spur one of several arms and smaller appenda
where our galaxy creates stars Astronomers name the spiral arms after tconstellation where they appear prominent NASAJPL983085CALTECHR HURT 983080SSC983085CALT8 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 914
F U N
Much of the Milky Way Galaxyand its structure remain hid-den to earthbound observersbecause dust chokes the spi-ral arms Itrsquos like being in thewoods and trying to discernthe forestrsquos form
F A C T
F U N
In measuring distances inthe galaxy and the universeastronomers use a unitknown as the l ight-year Itrepresents the distance abeam of light travels in oneyear At 186000 miles persecond light traverses 59trillion miles in a year
F A C T
band of dust cuts through the Milky Way blocking light
om distant stars If not for all the dust the galaxyrsquos centerould shine brighter than the brightest star STEVE THORNTON
F U N
A century ago astronomersthought the Sun occupiedthe center of the galaxy Butcareful studies of globularclusters which orbit theMilky Wayrsquos center and tendto gather in the constellationSagittarius show we livehalfway to the edge
F A C T
F U N
How do astronomers knowa black hole resides at theMilky Wayrsquos center They havefound stars near the centralhub orbiting so fast that theymust be circling an invisibleobject containing 4 millionsolar masses
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1014
T
he collections of stars gas and dust known as galaxies
form the building blocks of the observable universe
Roughly 125 billion galaxies populate the cosmos and
they come in all shapes and sizes Astronomers divide
galaxies into three major categories spirals ellipticalsand irregulars A spiral has a broad disk containing clouds of gas
and dust and from two to several spiral arms a nuclear bulge of
old stars and a spherical halo that envelops both
Approximately one-third of spirals exhibit central bars mdash a
symmetric concentration of stars and sometimes gas and dust
that crosses the nucleus and connects with the outer spiral arms
(Recent studies show the Milky Way possesses a significant bar)
The diameters of spirals range from roughly 20000 to more
than 100000 light-years and they contain anywhere from sev-
eral billion to several hundred billion stars
Elliptical galaxies appear spherical or flattened in shape
They possess little of the gas and dust seen in the disks of spiralgalaxies so they donrsquot generate any new stars Ellipticals show
the widest range in size of any galaxy type Giant ellipticals can
span 1 million light-years and contain several trillion stars
dwarf ellipticals may be only a few thousand light-years across
and have millions of stars An important intermediate type of
galaxy has a disk like a spiral galaxy but contains no gas or dust
Astronomers call this type of galaxy a lenticular
Irregular galaxies donrsquot show any symmetry or organized
spiral structure The category exists basically as a catchall for
galaxies that donrsquot fit either the spiral or elliptical classification
Irregulars can be big containing up to 100 billion stars or as
small as dwarf ellipticals Astronomers think most irregulars
result from the collisions or mergers of two or more galaxies
The gravitational interactions disrupt normal spiral or elliptical
structure leaving behind a chaotic appearance
Most galaxies belong to groups with dozens of members or
to clusters with up to thousands of members The Milky Way
joins with the slightly larger Andromeda Galaxy to form the
cornerstones of the Local Group a collection of roughly 50 gal-
axies that spans several million light-years The vast majority of
Local Group galaxies are dwarf ellipticals and irregulars Small
groups generally have a few dozen member galaxies but clusters
can contain several thousand galaxies The Virgo cluster located
50 million light-years away is the nearest large cluster to Earth
Galaxies
in theuniverse
The Mice
NGC 4414
Multiple spiral arms wind out from the nucleus ofNGC 4414 Young blue stars throng the armswhile older redder stars populate the nuclearbulge NASATHE HUBBLE HERITAGE TEAM 983080STSCIAURA983081
The ldquoMicerdquo are two spiral galaxies in the process of merging Gravity haspulled material out of each to form long tails while compressed gas cloufuel new star formation NASAESATHE ACS SCIENCE TEAM
M87
A high-speed jet shoots from the heart of the giant
elliptical galaxy M87 (upper left) in the Virgo cluster A black hole of some3 billion solar masses drives this activity NASATHE HUBBLE HERITAGE TEAM 983080STSCIAUR
Giant elliptical galaxies M84 (left) and M86 (right)each contain a trillion stars These two dominate the central regionof the nearby Virgo cluster a collection of some 2000 galaxies NOAOAURA
M84 amp M86
10 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1114
NGC 68221760 kly
Ursa Minor Dwarf 225 kly
Draco Dwarf
248 kly MilkyWay
SMC189 kly
NGC 1852020 kly
NGC 1472460 kly
Andromeda Galax
2510 kly
Pinwheel Galaxy (M33)
2770 kly
Leo I818 kly
Leo II750 kly
Sextans
Dwarf 293 kly
LMC160 kly
The Local GroupThe Milky Way and Andromeda galaxiesrule the Local Group accounting for morethan half its mass Distances from our galaxy
are given in thousands of light-years (kly) ASTRONOMY ROEN KELLY
F U N
If you examine the brightestgalaxies some 75 percent arespirals 20 percent ellipticalsand 5 percent irregularsIncluding faint dwarfs skews
the numbers to 30 percentspirals 60 percent ellipticalsand 10 percent irregulars
F A C T
ndromeda Galaxyghtly bigger than the Milky Way the Andromeda Galaxy (M31) containsme 500 billion stars Located 25 million light-years away the galaxy can bempsed with the naked eye MICHAEL STECKER
he LMC
e Large Magellanicoud (LMC) is an irregular
axy about 160000ht-years from Earth Thedish cloud at top righthe Tarantula Nebula
e largest known regionstar formation LUKE DODD
F U N
The Large and SmallMagellanic Clouds (LMCand SMC) are satellite galax-ies to the Milky Way They liedeep in the southern sky andwere not seen by Europeansuntil Magellanrsquos around-the-world voyage
F A C T
F U N
The Andromeda Galaxy mayseem a good neighbor tothe Milky Way but it wonrsquotalways be so Astronomersthink that in approximately 5billion years our two galaxieswill collide and merge
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1214
H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1314
Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1414
Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 914
F U N
Much of the Milky Way Galaxyand its structure remain hid-den to earthbound observersbecause dust chokes the spi-ral arms Itrsquos like being in thewoods and trying to discernthe forestrsquos form
F A C T
F U N
In measuring distances inthe galaxy and the universeastronomers use a unitknown as the l ight-year Itrepresents the distance abeam of light travels in oneyear At 186000 miles persecond light traverses 59trillion miles in a year
F A C T
band of dust cuts through the Milky Way blocking light
om distant stars If not for all the dust the galaxyrsquos centerould shine brighter than the brightest star STEVE THORNTON
F U N
A century ago astronomersthought the Sun occupiedthe center of the galaxy Butcareful studies of globularclusters which orbit theMilky Wayrsquos center and tendto gather in the constellationSagittarius show we livehalfway to the edge
F A C T
F U N
How do astronomers knowa black hole resides at theMilky Wayrsquos center They havefound stars near the centralhub orbiting so fast that theymust be circling an invisibleobject containing 4 millionsolar masses
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1014
T
he collections of stars gas and dust known as galaxies
form the building blocks of the observable universe
Roughly 125 billion galaxies populate the cosmos and
they come in all shapes and sizes Astronomers divide
galaxies into three major categories spirals ellipticalsand irregulars A spiral has a broad disk containing clouds of gas
and dust and from two to several spiral arms a nuclear bulge of
old stars and a spherical halo that envelops both
Approximately one-third of spirals exhibit central bars mdash a
symmetric concentration of stars and sometimes gas and dust
that crosses the nucleus and connects with the outer spiral arms
(Recent studies show the Milky Way possesses a significant bar)
The diameters of spirals range from roughly 20000 to more
than 100000 light-years and they contain anywhere from sev-
eral billion to several hundred billion stars
Elliptical galaxies appear spherical or flattened in shape
They possess little of the gas and dust seen in the disks of spiralgalaxies so they donrsquot generate any new stars Ellipticals show
the widest range in size of any galaxy type Giant ellipticals can
span 1 million light-years and contain several trillion stars
dwarf ellipticals may be only a few thousand light-years across
and have millions of stars An important intermediate type of
galaxy has a disk like a spiral galaxy but contains no gas or dust
Astronomers call this type of galaxy a lenticular
Irregular galaxies donrsquot show any symmetry or organized
spiral structure The category exists basically as a catchall for
galaxies that donrsquot fit either the spiral or elliptical classification
Irregulars can be big containing up to 100 billion stars or as
small as dwarf ellipticals Astronomers think most irregulars
result from the collisions or mergers of two or more galaxies
The gravitational interactions disrupt normal spiral or elliptical
structure leaving behind a chaotic appearance
Most galaxies belong to groups with dozens of members or
to clusters with up to thousands of members The Milky Way
joins with the slightly larger Andromeda Galaxy to form the
cornerstones of the Local Group a collection of roughly 50 gal-
axies that spans several million light-years The vast majority of
Local Group galaxies are dwarf ellipticals and irregulars Small
groups generally have a few dozen member galaxies but clusters
can contain several thousand galaxies The Virgo cluster located
50 million light-years away is the nearest large cluster to Earth
Galaxies
in theuniverse
The Mice
NGC 4414
Multiple spiral arms wind out from the nucleus ofNGC 4414 Young blue stars throng the armswhile older redder stars populate the nuclearbulge NASATHE HUBBLE HERITAGE TEAM 983080STSCIAURA983081
The ldquoMicerdquo are two spiral galaxies in the process of merging Gravity haspulled material out of each to form long tails while compressed gas cloufuel new star formation NASAESATHE ACS SCIENCE TEAM
M87
A high-speed jet shoots from the heart of the giant
elliptical galaxy M87 (upper left) in the Virgo cluster A black hole of some3 billion solar masses drives this activity NASATHE HUBBLE HERITAGE TEAM 983080STSCIAUR
Giant elliptical galaxies M84 (left) and M86 (right)each contain a trillion stars These two dominate the central regionof the nearby Virgo cluster a collection of some 2000 galaxies NOAOAURA
M84 amp M86
10 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1114
NGC 68221760 kly
Ursa Minor Dwarf 225 kly
Draco Dwarf
248 kly MilkyWay
SMC189 kly
NGC 1852020 kly
NGC 1472460 kly
Andromeda Galax
2510 kly
Pinwheel Galaxy (M33)
2770 kly
Leo I818 kly
Leo II750 kly
Sextans
Dwarf 293 kly
LMC160 kly
The Local GroupThe Milky Way and Andromeda galaxiesrule the Local Group accounting for morethan half its mass Distances from our galaxy
are given in thousands of light-years (kly) ASTRONOMY ROEN KELLY
F U N
If you examine the brightestgalaxies some 75 percent arespirals 20 percent ellipticalsand 5 percent irregularsIncluding faint dwarfs skews
the numbers to 30 percentspirals 60 percent ellipticalsand 10 percent irregulars
F A C T
ndromeda Galaxyghtly bigger than the Milky Way the Andromeda Galaxy (M31) containsme 500 billion stars Located 25 million light-years away the galaxy can bempsed with the naked eye MICHAEL STECKER
he LMC
e Large Magellanicoud (LMC) is an irregular
axy about 160000ht-years from Earth Thedish cloud at top righthe Tarantula Nebula
e largest known regionstar formation LUKE DODD
F U N
The Large and SmallMagellanic Clouds (LMCand SMC) are satellite galax-ies to the Milky Way They liedeep in the southern sky andwere not seen by Europeansuntil Magellanrsquos around-the-world voyage
F A C T
F U N
The Andromeda Galaxy mayseem a good neighbor tothe Milky Way but it wonrsquotalways be so Astronomersthink that in approximately 5billion years our two galaxieswill collide and merge
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1214
H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1314
Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1414
Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1014
T
he collections of stars gas and dust known as galaxies
form the building blocks of the observable universe
Roughly 125 billion galaxies populate the cosmos and
they come in all shapes and sizes Astronomers divide
galaxies into three major categories spirals ellipticalsand irregulars A spiral has a broad disk containing clouds of gas
and dust and from two to several spiral arms a nuclear bulge of
old stars and a spherical halo that envelops both
Approximately one-third of spirals exhibit central bars mdash a
symmetric concentration of stars and sometimes gas and dust
that crosses the nucleus and connects with the outer spiral arms
(Recent studies show the Milky Way possesses a significant bar)
The diameters of spirals range from roughly 20000 to more
than 100000 light-years and they contain anywhere from sev-
eral billion to several hundred billion stars
Elliptical galaxies appear spherical or flattened in shape
They possess little of the gas and dust seen in the disks of spiralgalaxies so they donrsquot generate any new stars Ellipticals show
the widest range in size of any galaxy type Giant ellipticals can
span 1 million light-years and contain several trillion stars
dwarf ellipticals may be only a few thousand light-years across
and have millions of stars An important intermediate type of
galaxy has a disk like a spiral galaxy but contains no gas or dust
Astronomers call this type of galaxy a lenticular
Irregular galaxies donrsquot show any symmetry or organized
spiral structure The category exists basically as a catchall for
galaxies that donrsquot fit either the spiral or elliptical classification
Irregulars can be big containing up to 100 billion stars or as
small as dwarf ellipticals Astronomers think most irregulars
result from the collisions or mergers of two or more galaxies
The gravitational interactions disrupt normal spiral or elliptical
structure leaving behind a chaotic appearance
Most galaxies belong to groups with dozens of members or
to clusters with up to thousands of members The Milky Way
joins with the slightly larger Andromeda Galaxy to form the
cornerstones of the Local Group a collection of roughly 50 gal-
axies that spans several million light-years The vast majority of
Local Group galaxies are dwarf ellipticals and irregulars Small
groups generally have a few dozen member galaxies but clusters
can contain several thousand galaxies The Virgo cluster located
50 million light-years away is the nearest large cluster to Earth
Galaxies
in theuniverse
The Mice
NGC 4414
Multiple spiral arms wind out from the nucleus ofNGC 4414 Young blue stars throng the armswhile older redder stars populate the nuclearbulge NASATHE HUBBLE HERITAGE TEAM 983080STSCIAURA983081
The ldquoMicerdquo are two spiral galaxies in the process of merging Gravity haspulled material out of each to form long tails while compressed gas cloufuel new star formation NASAESATHE ACS SCIENCE TEAM
M87
A high-speed jet shoots from the heart of the giant
elliptical galaxy M87 (upper left) in the Virgo cluster A black hole of some3 billion solar masses drives this activity NASATHE HUBBLE HERITAGE TEAM 983080STSCIAUR
Giant elliptical galaxies M84 (left) and M86 (right)each contain a trillion stars These two dominate the central regionof the nearby Virgo cluster a collection of some 2000 galaxies NOAOAURA
M84 amp M86
10 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1114
NGC 68221760 kly
Ursa Minor Dwarf 225 kly
Draco Dwarf
248 kly MilkyWay
SMC189 kly
NGC 1852020 kly
NGC 1472460 kly
Andromeda Galax
2510 kly
Pinwheel Galaxy (M33)
2770 kly
Leo I818 kly
Leo II750 kly
Sextans
Dwarf 293 kly
LMC160 kly
The Local GroupThe Milky Way and Andromeda galaxiesrule the Local Group accounting for morethan half its mass Distances from our galaxy
are given in thousands of light-years (kly) ASTRONOMY ROEN KELLY
F U N
If you examine the brightestgalaxies some 75 percent arespirals 20 percent ellipticalsand 5 percent irregularsIncluding faint dwarfs skews
the numbers to 30 percentspirals 60 percent ellipticalsand 10 percent irregulars
F A C T
ndromeda Galaxyghtly bigger than the Milky Way the Andromeda Galaxy (M31) containsme 500 billion stars Located 25 million light-years away the galaxy can bempsed with the naked eye MICHAEL STECKER
he LMC
e Large Magellanicoud (LMC) is an irregular
axy about 160000ht-years from Earth Thedish cloud at top righthe Tarantula Nebula
e largest known regionstar formation LUKE DODD
F U N
The Large and SmallMagellanic Clouds (LMCand SMC) are satellite galax-ies to the Milky Way They liedeep in the southern sky andwere not seen by Europeansuntil Magellanrsquos around-the-world voyage
F A C T
F U N
The Andromeda Galaxy mayseem a good neighbor tothe Milky Way but it wonrsquotalways be so Astronomersthink that in approximately 5billion years our two galaxieswill collide and merge
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1214
H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1314
Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1414
Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1114
NGC 68221760 kly
Ursa Minor Dwarf 225 kly
Draco Dwarf
248 kly MilkyWay
SMC189 kly
NGC 1852020 kly
NGC 1472460 kly
Andromeda Galax
2510 kly
Pinwheel Galaxy (M33)
2770 kly
Leo I818 kly
Leo II750 kly
Sextans
Dwarf 293 kly
LMC160 kly
The Local GroupThe Milky Way and Andromeda galaxiesrule the Local Group accounting for morethan half its mass Distances from our galaxy
are given in thousands of light-years (kly) ASTRONOMY ROEN KELLY
F U N
If you examine the brightestgalaxies some 75 percent arespirals 20 percent ellipticalsand 5 percent irregularsIncluding faint dwarfs skews
the numbers to 30 percentspirals 60 percent ellipticalsand 10 percent irregulars
F A C T
ndromeda Galaxyghtly bigger than the Milky Way the Andromeda Galaxy (M31) containsme 500 billion stars Located 25 million light-years away the galaxy can bempsed with the naked eye MICHAEL STECKER
he LMC
e Large Magellanicoud (LMC) is an irregular
axy about 160000ht-years from Earth Thedish cloud at top righthe Tarantula Nebula
e largest known regionstar formation LUKE DODD
F U N
The Large and SmallMagellanic Clouds (LMCand SMC) are satellite galax-ies to the Milky Way They liedeep in the southern sky andwere not seen by Europeansuntil Magellanrsquos around-the-world voyage
F A C T
F U N
The Andromeda Galaxy mayseem a good neighbor tothe Milky Way but it wonrsquotalways be so Astronomersthink that in approximately 5billion years our two galaxieswill collide and merge
F A C T
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1214
H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1314
Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1414
Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1214
H
ow did the universe get to be the way it is today It
might seem a hopeless question at first at least until
scientists invent a time machine to take us back But
astronomers have invented such an instrument mdash they
call it a telescope Herersquos how it works Because lighttravels at a finite speed (186000 miles per second) the light we
receive on Earth left its place of origin some time ago The far-
ther we look into space the further we peer back in time
When astronomers first studied galaxies in the early 20th
century they found the farther a galaxy was from Earth the
faster it appeared to be moving away If you think of this expan-
sion as a movie and run it backward then all of the galaxies
must have been much closer together in the past This led to
the idea of the Big Bang mdash the theory that all matter in the uni-
verse started out together and then something triggered a rapid
expansion that continues today
But is there any proof of such an extraordinary beginningYes mdash lots of it Astronomers looking ever deeper into space
find that the universe was surprisingly different Instead of the
stately spiral and elliptical galaxies we see now there were lots
of galactic fragments that were colliding merging and creat-
ing general havoc The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies creating highly
luminous quasars All this action took place mdash in fact could
only take place mdash in a universe much smaller than todayrsquos
Radio astronomers have even discov-
ered the echo of the initial fireball
which has cooled to a few degrees
above absolute zero
An even more shocking discovery
came at the end of the 20th century
By looking at supernova explosions
in the distant universe astronomers
discovered that the blasts did not
appear as bright as expected The conclusion The universal
expansion is accelerating In essence a long-range repulsive
force must be driving the universe to expand at ever greater
speeds As the Scottish geneticist J B S Haldane once famously
said ldquoThe universe is not only queerer than we imagine it is
queerer than we can imaginerdquo
Probingthe distantuniverse
F U N
Galaxies do not spreadevenly but gather in clustersthat themselves form vastfilaments leaving huge voidsin between
F A C T
Abell 1689
Thousands of galaxies cluster together in Abell 1689 The concentratioluminous matter and dark matter (stuff we canrsquot see but which adds to gity) creates a fun-house mirror of arcs and wisps NASAESATHE ACS SCIENCE TEA
F U N
For decades astronomershave known that the universecontains lots we canrsquot seeThis mysterious dark mattersurrounds galaxies and bindsclusters Scientists suspectexotic subatomic particlesare the culprit
F A C T
12 Your guide to planets stars and galaxies
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1314
Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1414
Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1314
Dark ener
Atoms 5
Dark matter
F U N
Quasars mdash short for quasi-stellar radio sources mdash arethe brightest objects in theuniverse They radiate asmuch energy as an entire gal-axy from a volume no biggerthan our solar system
F A C T
F U N
Black holes are so dense thattheir gravity prevents evenlight from escaping Theyrange in size from monstersin galactic cores to star-sizedremnants of supernovaeSome scientists think thereeven may be mini-black holesleft over from the Big Bang
F A C T
Cosmic microwave background
The atoms that make up stars planets and us add up to just 5 percent of thuniverse Invisible dark matter makes up 23 percent more while the dark enthat drives the accelerating cosmos accounts for 72 percent ASTRONOMY ROEN K
The Great Attractor
The biggest concentration of matter in the nearby universe is a string ofhuge galaxy clusters known as the Great Attractor which is pulling the LoGroup and the Virgo cluster in its general direction EUROPEAN SOUTHERN OBSERV
The microwave background glows at a nearly constant temperature of 49deg above absolute zero Tiny variations in the glow are subtle density fluctuationthat gave rise to the galaxies and voids we see in the universe NASAWMAP SCIENC
What is the universe made of
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1414
Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14
8112019 Your Guide to Planets Stars and Galaxies
httpslidepdfcomreaderfullyour-guide-to-planets-stars-and-galaxies 1414
Y
ou can make a connection to the universe at large
on any clear night Simply head outside and look up
You donrsquot need binoculars or a telescope (although
they wonrsquot hurt) mdash all you need are your eyes The
simplest thing to do is trace the patterns of stars Inwinter look for the commanding figure of Orion the Hunter
Spring brings the Big Dipper probably the most recognizable
stellar group in the sky
In the summertime look for the Northern Cross and trace the
path of the Milky Way which appears most prominent this time
of year The Great Square of Pegasus beckons on autumn eve-
nings You can find these patterns and from them hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy
Next use ldquoThe Sky this Monthrdquo in the magazine to home in
on the most visually arresting events in a given month It could
be a nice meteor shower where you might see a ldquoshooting starrdquoevery minute or so Or maybe therersquoll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month) Or perhaps yoursquoll be lucky and
get to witness a solar or lunar eclipse After all in the world of
backyard astronomy the sky is the limit
You and theuniverse
Conjunctions
Solar eclipses
When worlds align observers can expect a treat Here the Moon passes
front of the Sun blocking the brilliant solar disk and revealing the coronadisplays are possible because by cosmic coincidence the Sun is 400 timesfarther from Earth than the Moon and 400 times bigger than the Moon D
Lunar eclipses
A ruddy Moon signals a total lunar eclipse when the Moon dips completelyinto Earthrsquos shadow Earthrsquos atmosphere acts like a filter scattering out blue
When a crescent Moon passes a bright star or planet during the twilighhours itrsquos a sight that thrills any skywatcher experienced or not RANDALL
Meteor showers
Meteors storm from the sky at speeds of up to 44 miles per second Fric-tion with Earthrsquos atmosphere heats the tiny dust particles until they flare
Annual showers
Name Peak date
Quadrantids Jan 3
Lyrids April 22
Eta Aquarids May 6
Perseids Aug 12
Orionids Oct 21
Leonids Nov 17
Geminids Dec 14