chapter 18 the universe sections 18.1-18.7. copyright © houghton mifflin company. all rights...

Chapter 18

The Universe

Sections 18.1-18.7

Copyright © Houghton Mifflin Company. All rights reserved. 18 | 2

The Universe

• When we use the term universe we are discussing the totality of all matter, energy, and space.

• In this chapter we will examine the stars, galaxies, and cosmology.– In particular we will discuss our sun and

the classification of the stars.– Galaxies are giant assemblages of stars.– Cosmology deals with the structure and

evolution of the universe.

Intro


Studying the Universe

• Stars and galaxies emit the full spectrum of electromagnetic radiation.

• For many years we were only able to gather information from one type of emitted electromagnetic radiation – visible light.

• In 1931 radio telescopes became operational, giving scientists the ability to start gathering information from other wavelengths.

Intro


Studying the Universe

• Unfortunately for the astronomers, the Earth’s protective atmosphere also absorbed much of the incoming radiation from the non-visible wavelengths.

• Scientists were soon devising ways to minimize or eliminate the Earth’s atmospheric filtering.

• Observatories were located on high mountains where the atmosphere is thin .

• Balloons, aircraft, satellites, and spacecraft were utilized to transport instruments to ever greater heights.

Intro


Astronomy ≠ Astrology

• Astronomy is the scientific study of the universe beyond the atmosphere of Earth.– Astronomers largely study the electromagnetic

radiation emitted by celestial bodies that eventually reaches their instruments.

• Astrology is the ‘study’ of how the position of planets/stars at the time of one’s birth may affect an individual’s personality and/or future.– Although many people closely follow their

astrological chart, there is no known scientific basis for a cause/effect relationship.

Intro


The Sun

• Star - a self-luminous sphere of hot gases, energized by nuclear reactions and held together by the force of gravity.

• The Sun is the nearest star to Earth.• The Sun is enormous in size relative to

the size of Earth.– The Sun’s diameter is approximately 4

times the distance between the Earth and the Moon.

Section 18.1


Basic Information about the Sun

Section 18.1


Structure and Composition of the Sun

• The Sun can divided into four concentric layers:– Core - the innermost part of the Sun where

nuclear fusion occurs– Photosphere - the “surface” that we see– Chromosphere – the layer of very hot

gases above the photosphere, also known as the Sun’s lower atmosphere

– Corona - the Sun’s outer atmosphere

Section 18.1


Radial Cross Section of the Sun

The boundary between any two layers is not sharply defined.

Section 18.1


Photosphere

• The photosphere is the bright and visible “surface” of the Sun.– Temperature of ~6000K– Composition of 75% H, 25% He, and 1% C, O, N,

Ne, and others

• When viewed close-up, the photosphere surface appears to have a ‘granular’ texture.

• Each granule is a hot spot caused by an individual convection cell bringing thermal energy to the surface.– Each granule is about the width of Texas.

Section 18.1


Sunspots

• Sunspots are huge patches of cooler, and therefore darker material on the surface of the Sun.– They are a distinctive feature of the photosphere.

• The late 1600’s were an unusually cold period on Earth, marked by an anomalously fewer number of sunspots.– In Europe this time was known as the Little Ice

Age.

• Apparently, there is a strong connection between solar activity, as displayed by sunspots, and global climates on Earth.

Section 18.1


Close-up View of a Sunspot on the Photosphere

Note the granular appearance of the Sun’s surface

Section 18.1


Very Large Sunspot GroupMarch-April 1947

Section 18.1


Sunspot Cycle

• The abundance of sunspots vary through an 11-year sunspot cycle.

• Each sunspot cycle begins with the appearance of a few sunspots near the 30o “N & S” latitudes.

• The number of sunspots slowly increase, with a maximum number in the middle of the cycle at around the 15o region.

• The number of sunspots then slowly taper off during the last half of the 11-year cycle.

Section 18.1


Sunspot Cycle

• Interestingly, sunspots also have magnetic polarity.

• Each 11-year cycle is similar except that the polarities are reversed.

• Therefore the 11-year sunspot cycle is actually a manifestation of a more fundamental 22-year magnetic cycle.

Section 18.1


Flares and Prominences

• Flares - bright explosive events that occur on the Sun’s surface.

• Prominences - enormous filaments of solar gases that arch out and over the Sun’s surface.– Prominences may extend hundreds of

thousands of km outward.– Since they occur along the outer edge of

the Sun, they are particularly evident during solar eclipses.

Section 18.1


A Major Solar ProminenceFlares can also be seen as the bright yellow areas

Section 18.1


Corona

• The corona is the pearly white halo or crown extending far beyond the solar disk– Visible during total eclipses of the Sun.

• The corona can be interpreted to be the Sun’s “outer atmosphere.”– Corona’s temperature is approximately

1,000,000 K.

• Prominences occur within the corona region of the Sun.

Section 18.1


Solar Corona


Solar Wind

• In the extreme high-temperatures of the corona protons, electrons, and ions are furnished with enough energy to escape the Sun’s atmosphere. This is called the solar wind.– These particles are accelerated enough to escape

the Sun’s tremendous gravitational pull.• The solar wind extends out from the Sun at

least 50 AU.• Heliosphere is the volume or “bubble” of

space formed by the solar wind.– Virtually all the material in the heliosphere

emanates from the Sun.

Section 18.1


Interior of the Sun

• The Sun’s interior is so hot that individual atoms cannot exist.– Continuous high-speed collisions result in the

separation of nuclei and electrons.• A fourth phase of matter, called a plasma, is

created where nuclei and electrons exist as a high-temperature gas.

• The temperature at the central core of the Sun is 15,000,000 K and the density is 150 g/cm3.– The innermost 25% of the Sun, the core, is where

H is consumed to form He.

Section 18.1


Interior of the Sun

• Moving outward from the core of the Sun, both the temperature and the density decrease.

• It is the nuclear fusion of H into He within the core that is the source of the Sun’s energy.

• In the Sun and other similar stars the nuclear fusion takes place as a three step process called the proton-proton chain.

Section 18.1


Proton-Proton Chain or PP ChainEach reaction liberates energy by the conversion of mass.

Section 18.1


Proton-Proton Chain

• In the net reaction of the PP Chain, four protons form a He nucleus, two positrons, two neutrinos, and two gamma rays.

• The amount of energy released by the conversion of mass conforms with Einstein’s equation, E = mc2 .

Section 18.1


Solar Longevity

• Approximately 6.00 X 1011 kg of H is converted into 5.96 X 1011 kg of He every second

• With a total mass of 1030 kg of H, scientists expect the Sun to continue to radiate energy from H fusion for another 5 billion years

Section 18.1


Solar Constant

• The Earth only receives a very small percent of the Sun’s radiated energy.

• Every second the Earth receives approximately 1.4 x 103 W/m2, a value known as the solar constant.

• Even a very small variation (+/- 0.5%) in the solar constant would have disastrous effects on our planet’s biota.

Section 18.1


The Celestial Sphere

• Celestial sphere – the huge imaginary sphere of the sky on which all the stars seem to appear

• During any given night, the great dome of stars appear to progressively move westward, from an observer’s vantage point on Earth.

• North celestial pole (NCP) – the point in the Northern Hemisphere that the stars seem to rotate around– Polaris or the “North Star”

• South celestial pole (SCP) – the point in the Southern Hemisphere that the stars seem to rotate around

Section 18.2


Star Trails

• The stars’ apparent movement is due to the Earth’s actual west-to-east rotation.

• The NCP & SCP are simply extensions of the Earth’s rotational axes.

North Celestial Pole (NCP)

Section 18.2


The Celestial Sphere

• Key planes and points on the celestial sphere include the ecliptic, the celestial equator, the north celestial pole, and the south celestial pole.

• Celestial equator – the extension of the Earth’s equator

• Ecliptic – the apparent annual path of the Sun on the celestial sphere– Due to the Earth’s orbit around the Sun,

NOT because of the Sun’s movement

Section 18.2


Celestial Sphere

• Because of the 23.5o tilt of the Earth’s rotational axis, the ecliptic & celestial equator are inclined to one another, these two planes intersect at only two points.

Section 18.2


Equinoxes

• The two points at which the ecliptic and the celestial equator intersect are called the equinoxes.

• As the Sun appears to be moving southward on the ecliptic, it intersects the celestial equator around September 23, at a point called the autumnal equinox.

• About 6 months later, as the Sun appears to be moving northward on the ecliptic, it intersects the celestial equator around March 21, at a point called the vernal equinox.

Section 18.2


Celestial Sphere

• On these two days the Sun is directly over the equator and every location on earth receives an equal amount of daylight and darkness (12 hours.)

Section 18.2


Solstices

• The two solstices also mark significant points on the solar ecliptic.

• Summer solstice - the point along the ecliptic where the Sun’s location is the farthest north

• The winter solstice is the point along the ecliptic where the Sun’s location is the farthest south.

Section 18.2


Celestial Prime Meridian

• Celestial prime meridian – the half-circle created by the intersection of the NCP, the vernal equinox, and the SCP

• The celestial prime meridian is therefore a reference line or starting line used to measure celestial longitude.

Section 18.2


Celestial Longitude & Latitude

• The measure of celestial longitude is called the right ascension (RA) of the star or galaxy.

• The RA is the position of the star/galaxy to the east of the celestial prime meridian.– Measured in “hours, minutes, and seconds” with

the full circle having a total of 24 hours

• The declination (DEC) of a star or galaxy is an angular measurement (in degrees, minutes, and seconds) north or south of the celestial equator.

Section 18.2


Celestial Longitude (RA) and Latitude (DEC)

• Maximum RA is 24 hours and maximum DEC would be either +90o or –90o

Section 18.2


Celestial Distance

• The distance to most celestial bodies is measured in astronomical units (AU), light-years, or parsecs.

• Astronomical unit (AU) – the mean distance between the Earth and the Sun (1.5 x 108 km)

• Light-year (ly) – the distance light travels through a vacuum in one year (9.5 x 1012 km)

• Parsec (pc) – distance to a star when the star exhibits a parallax of 1 second of arc

(3.26 ly = 3.09 x 1013 km)

Section 18.2


Annual Parsec of a Nearby Star

• As a viewer’s position on Earth changes throughout the year, the position of a nearby star appears to shift, relative to more distant stars.

• This apparent annual motion of the star is called parallax.

• The closer the star, the greater the shift.

Section 18.2


Observing Parallax

• Parallax cannot be detected with the unaided eye.

• Friedrich Bessel, a German astronomer, was the first to observe stellar parallax.

• Even for the very closest stars, parallax is very small.– The parallax angle for the closest stars is the width

of a dime at a distance of 6 km!

• Only stars within about 300 ly can have their distance determined by parallax measurements.

Section 18.2


Stars

• Recall that stars are composed mostly of H, with some He, and far lesser amounts of other elements.

• They exist as huge plasma spheres in which nuclear fusion of H to He produces enormous amounts of energy that is emitted.

• Although star masses can vary considerably, most have a mass between 0.08 – 100 solar masses.– One solar mass = mass of our Sun

Section 18.3


Stars

• Most stars are part of a multiple-star system, unlike our Sun that is a single star.

• Binary stars – stars that consist of two close stars each orbiting about their shared center of mass– Most stars are part of a binary system.

• There are also star systems with three or more closely spaced stars but these are far less frequent.

Section 18.3


Constellations

• Constellations – prominent groups of stars that appear as distinct patterns to an observer on Earth– Most of the constellation names that we use can

be traced back to early Babylonian or Greek civilizations.

– Many other cultures have other names for these and different arrangements of the stars.

• Other than their peculiar patterns, constellations have no physical significance.

Section 18.3


Constellations

• Despite their ‘non-science’ significance, modern astronomers use the constellations to formally partition off the celestial sphere.

• In similar fashion to the individual stars, the constellations also appear to move westward across the sky each night– Due to Earth’s rotation.

• Constellations also exhibit an annual cycle of movement due to the tilt of the Earth and its annual orbit around the Sun.

Section 18.3


Asterisms

• Asterisms – familiar groups of stars that only comprise part of a constellation, or perhaps parts of two constellations

• The Big Dipper, the Little Dipper, and the Summer Triangle are each an example of an asterism.

Section 18.3


Ursa Major & Ursa Minor

• Note that the Big Dipper and the Little Dipper are each only a part of the larger named constellations.

• Polaris (the North Star) is located at the end of the handle on the little dipper.

Section 18.3


Celestial Magnitude

• The Greek astronomer and mathematician Hipparchus was a dedicated observer of the stars.

• He compiled the first star catalog by measuring the location and assigning apparent brightness magnitudes to more than 800 stars.

• Apparent magnitude – the brightness of any celestial object as observed from Earth– First magnitude stars were the brightest and sixth

magnitude stars were barely visible to the unaided eye.

Section 18.3


Apparent Magnitude

• The apparent magnitude scale in use today is a modified version of Hipparchus’ scale.– Instruments are used to accurately measure and

quantify the apparent magnitudes.

• The modern scale is constructed such that a difference of 5 magnitudes represents a difference of 100 in apparent brightness.– Therefore a first-magnitude star appears 100

times brighter than a sixth-magnitude star.

Section 18.3


Apparent Magnitude of other Celestial Bodies

• The brightest celestial body is the Sun. – The Sun has an apparent magnitude of –27.

• The full moon is the second brightest celestial body.– The full moon has an apparent magnitude of –13.

• Venus is the brightest planet.– Venus has an apparent magnitude of –4.

• Sirius is the brightest star. (8.7 ly in distance)– Sirius has an apparent magnitude of –1.

Section 18.3


Absolute Magnitude

• Obviously, a star’s distance from Earth tremendously affects its apparent brightness.

• Absolute magnitude – the brightness a star would have if it were placed 10 pc (32.6 ly) from Earth

• Our Sun, for example, has an absolute magnitude of +5.

• If we know the distance to a star and its apparent magnitude, the absolute magnitude can be calculated.

Section 18.3


The H-R (Hertzsprung-Russell) Diagram

• The H-R Diagram results from plotting the stars’ absolute magnitudes versus the temperatures of their photospheres.

• Most stars become brighter as they get hotter.

• These stars plot as a narrow diagonal band in the diagram.

• The hottest (and generally brightest) stars are blue. The coolest (and generally least bright) stars are red.

Section 18.3


Hertzsprung-Russell Diagram

Note that some stars do not fall along the main sequence.

Section 18.3


Stars off the H-R Main Sequence

• Several types of star do not fall on the main sequence.

• Red giants – very large stars that are cool, yet still very bright– The very brightest are called red

supergiants.

• White dwarfs – stars that are very hot, yet are dim due to their small size

Section 18.3


Spectral Analysis of the Stars

• Spectral analyses of stars shows that even the most distant stars contain the same elements we find in our solar system.

• However, there are variations in the spectra depending on the temperature of the individual star’s photosphere.

• Therefore, the patterns of the absorption lines in the spectra can be used to determine both the composition and the surface temperature.

Section 18.3


Spectral Classes• In the H-R diagram, stars have been placed

in seven spectral classes -- O,B,A,F,G,K,M.– Note the horizontal axis of the diagram.

Section 18.3


Spectral Classification

• Our Sun falls close to the middle of the main sequence and is a class G star.

• Sirius is quite a bit hotter than our Sun and is a class A star.

• Astronomers have found that the majority of stars are small, cool, class M stars, also called red dwarfs.– Proxima Centauri is the closest star to

Earth (besides our Sun) and is a red dwarf or class M star.

Section 18.3


Interstellar Medium

• Interstellar medium – gases and dust that is distributed amongst the stars

• The gases consists of about 75% H, 25% He, and a trace of heavier elements by mass.

• The dust (only about 1% of the interstellar medium) consists primarily of C, Fe, O, & Si.– About the size of particles in smoke

Section 18.4


Nebulae

• The gases and dust do not appear to be distributed uniformly throughout space.

• Nebulae – concentrations of cool, dense clouds of gases and dust

• Two major types of nebulae exist:

- bright nebulae

- dark nebulae

Section 18.4


Bright Nebulae – Two Types

• Bright nebulae can, in turn, be divided into:– Emission nebulae – energy from nearby

stars ionize the hydrogen gas resulting in fluorescence

– Reflection nebulae – dust within the nebulae reflect and scatter starlight, giving off a characteristic blue color

Section 18.4


The Great Nebula in Orion (M42)An Emission Nebula

Section 18.4


Dark Nebulae

• Dark nebulae – produced by the obstruction of a relatively dense cloud of interstellar dust

• Dark nebulae are visible as relatively dark areas as they are framed against some type of light-emitting region behind them.

• The most famous of the dark nebulae is located in the constellation Orion.

• It appears like the head of a horse, back-lit by a bright emission nebula.

Section 18.4


The Horsehead NebulaA Dark Nebula

Section 18.4


General Evolution of a Low-Mass Star

• Stars begin to form as interstellar material (mostly H) gathers together. (accretion)

• The accretion of the interstellar material may be due to several factors:– Gravitational attraction of the material– Radiation pressure from nearby stars– Shockwaves from exploding stars

• A protostar forms as the interstellar material condenses and the temperature rises.

Section 18.4


General Evolution of a Low-Mass Star

• As the protostar continues to condense, the temperature continue to rise.

• As the temperature continue to rise, the thermonuclear reaction begins in which H is converted to He. (fusion)– Recall the reaction below:

Section 18.4


A Star is Born

• When thermonuclear fusion begins, this is the time a star is actually born.

• It moves onto the H-R main sequence, in a position determined by its temperature and brightness.– Ultimately both temperature and brightness

are dependent upon the star’s mass.

Section 18.4


Stellar Evolution on the H-R Diagram

Section 18.4


Steller Evolution

• As the star continues to fuse H into He, it remains on the H-R main sequence.

• The period of time that a star remains on the main sequence can vary widely.

• A low-mass star, such as our Sun, will remain on the main sequence for 10 billion years.

• A very low-mass star may stay on the main sequence for trillions of years.

• High-mass stars fuse fuel fast, and only remain on the main sequence a few million years.

Section 18.4


Stellar Evolution

• As H in the core continues to convert to He, the core begins to contract and heat up even more.

• This extra heat eventually causes the H in the surrounding shell to proceed more rapidly, which in turn destabilizes the star’s hydrostatic equilibrium, resulting in expansion.

• At this point, when the star expands, it enters the red-giant phase, and moves off the main sequence.

Section 18.4


Red-Giant Phase

• When our Sun moves into its red-giant phase (several billion years from now) it will swell and engulf Mercury and possibly Venus.

• The core of a red-giant eventually becomes so hot that He will fuse into C and perhaps will create elements up through Ne.

• Nucleosynthesis - the creation of elemental nuclei inside stars.

• In high-mass red supergiants nucleosynthesis may continue all the way up to Fe.

Section 18.4


Variable Star

• Variations occur in the star’s temperature and brightness during the red-giant phase.

• For a short time it becomes a variable star, as its position on the H-R diagram moves left.

Section 18.4


Planetary Nebula

• During and after the variable star stage the star becomes very unstable, resulting in the outer layers being blown off forming a planetary nebula.

• Planetary nebula have nothing to do with planets.– Early and fuzzy photographs of planetary nebula

reminded astronomers of an evolving solar system.

• The expelled material diffuses into space, providing material for future generations of stars.

Section 18.4


The Eskimo Nebula: A Planetary Nebula

Note the visible white dwarf in the center

Section 18.4


White Dwarf

• Following the formation of a planetary nebula the remaining core is called a white dwarf.

• Thermonuclear fusion no longer occurs in a white dwarf and it slowly cools by radiating its residual energy into space.

• White dwarfs are not very bright and fall low on the H-R diagram.– A single teaspoon a matter in a white dwarf

may weigh 5 tons!

Section 18.4


Sirius – a Binary Star

• Sirius A is a class-A main sequence star.

• Sirius B is a white dwarf.

Section 18.4


Brown Dwarfs

• During stellar evolution, what if a protostar cannot sustain thermonuclear fusion due to its small mass?

• In this situation the protostar would not progress to the star phase and would result in a “failed star” or a brown dwarf.

• Obviously brown dwarf are very dim.• The first brown dwarf was not discovered until

1996. – Gleise 229B– About 100 brown dwarfs have now be found.

Section 18.4


Evolution of a low-mass starProtostar, main-sequence star, red giant, planetary nebula, and white dwarf

Section 18.4


Nova

• Nova – a white dwarf that temporarily and suddenly increases in brightness– Although the word nova mean “new star,” it

is not.

• Novas result from a nuclear explosion on the surface of a white dwarf.– This explosion is caused by small amounts

of material falling onto the white dwarf’s surface from its much larger binary companion.

Section 18.5


Supernova

• Supernova – the gigantic and catastrophic explosion of a star– Large amounts of material and radiation

are thrown off as a result of this explosion.

• There are two types of supernovas:– Type I supernovas - result from the

destruction of a white dwarf containing a carbon-oxygen core.

– Type II supernovas - result from the collapse of an iron core of a red supergiant.

Section 18.5


Supernova

• The Type I and Type II supernovae can be distinguished by their different spectral signatures:

• Type I supernova do not have hydrogen spectra nor do they emit radio waves.

• Type II supernova display hydrogen spectral lines and emit radio waves.

Section 18.5


The Crab NebulaType II Supernova

Section 18.5


Crab Nebula

• The Crab Nebula is the best known and one of only three supernovae that have been recorded in the Milky Way Galaxy.

• The supernova that resulted in the Crab Nebula was reported in Chinese and Japanese records around 1054.

• During the first two weeks of this supernova’s appearance, the night sky was bright enough to read by.

Section 18.5


Nucleosynthesis

• The energy and neutrons emitted during a supernova explosion result in the nucleosynthesis of the elements heavier than iron.

• Other than H and He, all of the elements up to Fe are thought to be made during normal fusion processes in stars of various masses.

• Elements past Fe are thought to be generated exclusively during supernova explosions.

Section 18.5


Neutron Stars

• When the nuclear fuel interior of a red supergiant is depleted, the interior undergoes a catastrophic collapse to form a small neutron star.– Approximately 20 km in diameter

• As a result of this inner collapse, the outer layers also collapse, bounce off the rigid inner core, and expand into space, resulting in an ever expanding supernova, leaving behind the small core (neutron star.)

Section 18.5


Neutron Stars

• The name “neutron star” comes from electrons and protons combining to form neutrons.

• Neutron stars are extremely dense – in the order of 1 billion tons per teaspoon.

Section 18.5


Pulsar

• Any angular momentum that the original star had must be maintained.

• Therefore, the small size of the neutron star dictates that it may spin rapidly.

• A pulsar emits radio waves that are detected on Earth in regular pulses apparently due to its rapid rotation.– Each pulsar has a constant period between 0.03 - 4s.

• Pulsars are interpreted to have a magnetic axis which is tilted relative to its magnetic axis.

Section 18.5


Rotating Neutron Star Model

• As the neutron star rotates the radiation beam (caused by the magnetic field) hits the Earth periodically.

• On Earth the radiation beam is detected in “pulses,” but actually is due to rotation.

Section 18.5


Very Large Array (VLA), New MexicoDesigned to detect radio-wave images of distant celestial objects

Section 18.5


The Crab Nebula

The pulsar at its center spins 30 time/second.This nebula also emits pulses of visible light.

Section 18.5


Black Holes

• Black hole – a celestial object so dense that the escape velocity is equal to or greater than the speed of light– Gravity’s ‘final’ victory over all other forces– Not even light cannot escape from its surface

• Black holes are thought to result from a supernova explosion when the core remaining is greater than ~3 times the mass of the Sun.

• The star’s matter continues to condense down to an extremely small point, a singularity.

Section 18.5


Configuration of a Nonrotating Black HoleWithin the event horizon boundary (the Schwarzschild radius R), the escape velocity is equal to or greater than the speed of light. Therefore no matter or radiation can escape.

Section 18.5


Detection of a Black Hole

• The event horizon of a black hole represents a one-way boundary: anything can enter but nothing can escape.

• If nothing, not even electromagnetic radiation, can escape a black hole then how can one be detected?

• One way is the detection of x-ray emission in the vicinity of a black hole.

• Captured gases, just outside the event horizon emit x-rays and form an accretion disk.

Section 18.5


Class O Star and Black Hole - Cygnus X-1

The x-rays that are being generated are apparently due to an accretion disk of gases being captured by the black hole.

Section 18.5


High-Mass vs. Low-Mass Stars

• High-mass stars and low-mass stars form initially in similar manners.

• High-mass stars are hotter and brighter than low-mass stars.– High-mass star move onto the main

sequence at higher points.

• High-mass stars do not stay on the main sequence as long as low-mass stars, due to their higher rate of thermonuclear fusion.

Section 18.5


Evolution of a High-Mass Star

When high-mass stars moves off the main sequence they become red supergiants and eventually explode as Type II supernovae. Much of the material is scattered into space leaving behind a neutron star or black hole.

Section 18.5


Mass and Stellar Evolution

• As we have learned in sections 18.4 & 18.5, the initial mass that a celestial object acquires during its formation is very important in determining its eventual fate.

• But the final mass left over after any stellar material is ejected turns out to be the conclusive determinant.

Section 18.5


The Fate of Celestial Objects

Section 18.5


Galaxies

• Galaxy – a very large aggregate of stars, gas, and dust held together by their mutual gravitational attraction

• Galaxies are considered to be fundamental components of the universe.

Section 18.6


Classification of Galaxies

• Edwin P. Hubble (1889 – 1953) established a system to classify galaxies into four types:– Elliptical– Normal spiral– Barred spiral– Irregular

• Hubble initially studied law under a Rhodes Scholarship but switched professions and completed his Ph.D. in astronomy in 1917.

Section 18.6


Elliptical Galaxies

• Elliptical galaxy – stars are arranged into a spherical or elliptical shape

• An elliptical galaxy does not have extending curved “arms.”

Section 18.6


Elliptical Galaxies in the Virgo Cluster

Section 18.6


Normal Spiral Galaxies

• Normal spiral galaxy – consists of two more-or-less definable regions

• The central nuclear bulge is the region where vast numbers of stars are tightly bunched together.

• Extending out from the nuclear bulge is a curved or spiral disk also containing numerous stars.

Section 18.6


Andromeda Galaxy (M31)Normal spiral galaxy

Section 18.6


Whirlpool Galaxy (M51) Normal spiral galaxy

Section 18.6


Sombrero Galaxy (M104) Normal spiral galaxy

Section 18.6


Barred Spiral Galaxies

• Barred spiral galaxy – two broad expansions (“bars”) extend out from opposite side of the nuclear bulge before the arms start to curve from the outer boundaries of the bars

Section 18.6


Barred Spiral Galaxy (NGC 1300)

Section 18.6


Irregular Galaxy• Irregular galaxy – a galaxy with no geometric

shape

Large Magellanic Cloud (LMC) & Small Magellanic Cloud (SMC)

Section 18.6


Galaxy Facts

• Several hundred thousand galaxies have been identified.

• Astronomers estimate there are 125 billion galaxies in the universe.

• In order of abundance:– Elliptical galaxies are a majority of the galaxies.– Normal spiral galaxies greatly outnumber barred

spiral galaxies.– Irregular galaxies comprise only about 3% of the

galaxies.

Section 18.6


Galaxy Distances

• The nearest galaxy to the Milky Way is Sagittarius Dwarf - About 50,000 ly.– Discovered in 1994– Sagittarius Dwarf is a small elliptical galaxy being

pulled into the gravity of the Milky Way.

• Second nearest galaxy to the Milky Way is LMC, at about 160,000 ly.

• SMC is the third closest to the Milky Way, at about 200,000 ly.

• The Andromeda Galaxy (M31) is about 2.2 ly from the Milky Way.

Section 18.6


The Milky Way Galaxy

• The Milky Way is a normal spiral galaxy, so-named because ancient people thought it appeared like a trail of spilled milk on a clear night.

• The broad band that we observe in the night sky is a view into the plane of the galactic disk.

• Although the unaided eye can distinguish about 6000 stars in the Milky Way, astronomers estimate there are actually between 100-200 billion.

Section 18.6


The Milky Way GalaxyFace-on and edge-on Views of our Galaxy

Section 18.6


Milky Way GalaxyImage obtained by Cosmic Background Explorer satellite (COBE)

Section 18.6



• Globular Clusters – “small” globular clusters of a few 10,000’s of stars– These cluster form a halo around the outside of

the Milky Way.

• In 1917 Harlow Shapley mapped the locations of the 93 then-known globular clusters and found that our solar system was not their center.

• The center of these globular clusters was about 28,000 ly away – marking the nuclear bulge of the Milky Way Galaxy.

Section 18.6



• Shapley correctly inferred that our solar system was about 30,000 ly away from the center of the nuclear bulge.

• Dimensions of the Milky Way galaxy are:– 100,000 ly in diameter– 2,000 ly is thickness of disk– 20,000 ly is the thickness of the nuclear bulge

• Accumulating evidence suggests that a massive black hole is located at the core of nearly every galaxy’s nuclear bulge. Scientists think our galaxy’s supermassive black hole has a mass of ~3.7 million solar masses.

Section 18.6


Quasars

• Quasar – short for quasi-stellar radio sources• The most distant objects in the universe,

perhaps the bright centers of distant galaxies– One quasar is thought to be 13 billion ly away.

• They are the source of huge amounts of electromagnetic radiation.

• This large release of energy is probably due to material as it orbits and is drawn into a supermassive black hole.

Section 18.6


Quasar 3C 273

• Thought to mark the location of supermassive (about 1 billion solar masses) black hole

Section 18.6


Looking Back in Time: Distant Stars and Galaxies

• The farther we look into space, the further we are looking back in time.

• Photographs taken today, are using light that may have left the star or galaxy thousands, millions, or even billions of years ago.

• When we look at a quasar that is 13 billion ly away, that light left the quasar 13 billion years ago, when the universe was very young.– Most astronomers think the universe is 14 billion

years old.

Section 18.6


Local Group, Clusters, & Superclusters

• Local Group – name given a group of almost 40 galaxies that includes the Milky Way, Andromeda, and M33– All within about 3 million ly from us and

gravitationally bound to each other

• Cluster – astronomers partition off the universe into volumes that range from 3 to 15 million ly in diameter

• Supercluster – clusters of clusters

Section 18.6


The Local Group of Galaxies

Composed of three dominant galaxies (Milky Way, Andromeda, & M33) and about 35 smaller galaxies

Section 18.6


Dark Matter

• Dark matter – the as yet unidentified non-luminous matter in the universe

• Analysis of the speed of revolution and the gravitational behavior of the stars and galaxies indicates that the galaxies contain much more matter than can be detected

• ~90% of the matter cannot be detected by any wavelength of electromagnetic radiation

• It is postulated that the Milky Way galaxy is embedded within a sea of dark matter that may extend outward for 150,000 ly

Section 18.6


Dark Matter

• Dark matter in the universe may consist of neutrinos, unknown black holes, brown dwarfs, neutron stars, and other potentially high-mass material

• MACHOs (massive compact halo objects) are hypothetical, but possible contributors to the dark matter

• WIMPs (weakly interacting massive particles) are exotic subatomic particles

Section 18.6


Hubble & Spectrum Shifts

• Hubble examined the spectral shifts of different galaxies.

• During his investigations he noted that some shifts were blue and some red.– The blue shift indicates that the galaxy is

moving toward us.– The red shift indicates that the galaxy is

moving away from us.

Section 18.7


Expanding Universe

• Hubble noticed that far-away galaxies all exhibited red shifts.

• He also noted that the farther away the galaxy, the larger the red shift.

• As a result, Hubble concluded that the universe must be expanding.

Section 18.7


Hubble’s Law

The greater the distance, the greater the recessional velocity (calculated from the observed red shift)

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Hubble’s Law

• Hubble’s Law – the recessional speed of a distant galaxy is directly proportional to its distance from the Milky Way galaxy

• v = Hd– v = recessional velocity of the galaxy– d = distance away from us– H = the Hubble constant

• H has not been precisely established yet but lies somewhere between 50 and 80 km/s per million parsecs, depending on what galaxies are being observed and what experimental techniques are used.

Section 18.7


Cosmology

• Cosmology – the study of the structure and evolution of the universe

• By detecting and analyzing electromagnetic radiation from these galaxies, astronomers can determine the structure of the universe.

• From work accomplish thus far, most galaxies occur in clusters and linear filaments.– One long broad concentration of galaxies is known

as the “Great Wall.”

Section 18.7


Distribution of GalaxiesGalaxies do not appear to be evenly distributed in space

Section 18.7


Consequence’s of Hubble’s Law

• What does it mean if all the other galaxies are all receding from away from us?

• We must be at the center of the universe – No!

• Almost all astronomers agree that we live in an ever expanding universe.

• Therefore, every galaxy is receding from every other galaxy.– An observer, from anywhere in the universe would

witness the same phenomena – receding galaxies.

Section 18.7


Rising Bread Analogy of the Expanding Universe

• The raisins represent galaxies and the dough represents space. As the loaf of bread rises, every raisin recedes from every other raisin.

• The greater the initial distance from a given raisin, the faster and farther the other raisins move

Section 18.7


Ever Expanding Universe?

• Earlier theories predicted that gravity would eventually rein in the expansion.

• Recent evidence points toward the universe expanding forever.

• In fact recent data suggests that the expansion is actually accelerating.

• Some astronomers have proposed a new form of energy – dark energy – to explain the acceleration.

Section 18.7


Dark Energy

• Dark Energy – a mysterious cosmic force causing the universe to accelerate

• Dark energy is not matter or radiation but it carries energy.

• Dark energy has a repulsive effect on the universe and causes empty space to expand.


The Big Bang

• While the galaxies are all moving away from each other at the present time, they must have been closer together in the past.

• The universe must have been much more compressed.

• In fact most astronomers think that the universe began as a small, hot, extremely dense entity that rapidly expanded (exploded) about 14 billion years ago – the Big Bang.

Section 18.7


The Big Bang – Progression of Events

• With our present knowledge of subatomic particles and the structure of the universe we can extrapolate back to about 10-43 second of the Big Bang.

• Initially the universe was filled with protons.• Within the first 4 seconds protons, neutrons,

and electrons formed.• D and He nuclei formed in the first 3 minutes.• After about ½ million years gravity began

forming galaxies and stars.

Section 18.7


The Big Bang – Broad Acceptance

• Experimental evidence supports the Big Bang in three major areas:

• Cosmological redshift – galaxies today have a redshift in their spectrum lines

• Cosmic microwave background – microwave radiation that fills all space and is thought to represent the redshifted glow from Big Bang

• There is a H to He mass ratio of 3 to 1 in the stars and interstellar material, as predicted by the Big Bang model.

Section 18.7


Big Bang - Standard & Inflationary Models

• With a doubt, our current knowledge of the formation of the universe is incomplete.

• In 1980 the standard model was modified. • The “inflationary model” proposes that the

universe consists of a large number of separate regions – formed at about 10-35 second.

• We can only detect information from one of these regions – our “universe.”

Section 18.7


Inflationary Model of the Big Bang

• Much greater expansion took place between 10-35 second and 10-30 second than in the standard model.

Section 18.7


Inflationary Model

• If the inflationary model is correct the entire universe is gigantic and the portion that we can observe is only a tiny fraction of the whole.

• At the present time the inflationary model is the best theory to explain the origin of the universe.

Section 18.7

chapter 18 the universe sections 18.1-18.7. copyright © houghton mifflin company. all rights...

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