fy12 astronomy

135
3. Astronomy ose you watch the sun set over a calm ocean e lying on the beach, starting a stopwatch j he top of the sun disappears. You then stand ating your eyes by a height h=1.7m, and stop h when the top of the sun again disappears. he elapsed time on the watch is 11.1 s, what he radius of the Earth? the oldest science, and the newest

Upload: bionioncle

Post on 10-May-2015

213 views

Category:

Documents


6 download

TRANSCRIPT

Page 1: Fy12 astronomy

3. Astronomy

Suppose you watch the sun set over a calm oceanwhile lying on the beach, starting a stopwatch justas the top of the sun disappears. You then stand,elevating your eyes by a height h=1.7m, and stop thewatch when the top of the sun again disappears.If the elapsed time on the watch is 11.1 s, whatis the radius of the Earth?

the oldest science, and the newest

Page 2: Fy12 astronomy

Mercury: sao Thủy, Venus: sao KimMars: sao Hỏa, Jupiter: sao MộcSaturn: sao Thổ, Uranus: Thiên vương tinhNeptune: Hải vương tinh

3.1 The starry sky

Page 3: Fy12 astronomy

• 100000ly across• Solar system:790000km/h220mil years – onetrip

The solar system in the Milky Way galaxy

Page 4: Fy12 astronomy
Page 5: Fy12 astronomy

Aristotle (340 B.C.) ‘On the Heavens’- The earth’s shadow in an eclipse was round- the North Star appeared lower in the sky in the south- the sails of a ship come into view before the hull

Aristarchus (310 B.C.)-Moon-Sun angle at half moon is 87o the Sun is 20 times farther from us than is the Moon -Lunar eclipse: The diameter of the Earth is 3 times larger than that of the Moon

Page 6: Fy12 astronomy

1, 87 ,

cos

18 20

correct value 390

oS

L

S

LS

L

Page 7: Fy12 astronomy

Eratosthenes (276 B.C.)circumference of the Earth36690km – 40075kmHipparchus (146 B.C.)-Star chart 850 entries-brightness-magnitudePtolemy (100), 765-> Arabic, a few centuries later, Muslim Spain->Latin -> Church dogma

Einstein, at age 16: ‘What would the world look like if I weresitting on a beam of light, moving at the speed of light’

Page 8: Fy12 astronomy

Location on Earth

• latitude lines• longitude lines(meridian lines)

Page 9: Fy12 astronomy
Page 10: Fy12 astronomy

Celestial sphere

Constelations

Page 11: Fy12 astronomy
Page 12: Fy12 astronomy
Page 13: Fy12 astronomy

Earth’s seasons

• Earth’s equator remains tilted at about 23.5°• the northern and southern hemispheres experience opposite seasons• seasonal variations in the length of days and nights

Page 14: Fy12 astronomy

(a) autumnal equinox ; (b) summer solstice ; (c) vernal equinox ; (d) winter solstice

Equinoxes and solstices

Page 15: Fy12 astronomy

Tides

Page 16: Fy12 astronomy

3.2 Light and Telescopes

What is light?

Page 17: Fy12 astronomy
Page 18: Fy12 astronomy

Radiation laws: Wien’s law: max=0.3/T - cm, T – K

Stefan-Boltzmann law: F=T4 F – energy flux

Hottest stars: blue-whiteCoolest stars: red

Page 19: Fy12 astronomy

Astronomical observations: 2 atmospheric windows- optical/visible light- some infrared and radio windows dark sites, dry, thin, steady air

Page 20: Fy12 astronomy

Optical telescopes: refractive and reflective- size of a telescope = size of its aperture- Eye lens size 5mm- 150 mm telescope – 30 times eye lens size light-gathering power 900 times greater- 10m telescope – faint stars with brightness of a candleviewed from the moon

Galileo Galilei – 1609, refracting 50mm-telescopeBeginner’s – 60mm, largest – 1m - 1897

A refracting telescope

Page 21: Fy12 astronomy

Reflecting telescopes:

Page 22: Fy12 astronomy

Resolving power:depends directly on the size of the aperture andinversely on the wavelength of the incoming light

Magnifying power: ratio of the apparent size of an object seen through the telescope to its size when seen by the eye alone

Page 23: Fy12 astronomy

Useful magnification

Page 24: Fy12 astronomy

Telescope design and selection:Stability is essential

Refractors: • rugged and require less maintenance• image quality and resolution

Reflectors: • greater aperture for the price• easier to make at home• faintest, most distant objects• folded optics reduce the physical length• the primary mirror is supported from behind

Page 25: Fy12 astronomy
Page 26: Fy12 astronomy

planned for 2020

Page 27: Fy12 astronomy

Radio telescope:

• objects that emit powerful radio waves but little visible light• radio sources behind interstellar dust clouds• can be used in cloudy weather and during the daytime

Aperture synthesis: combines data from two or more telescopes to simulate one very large aperture

Page 28: Fy12 astronomy

Radio astronomy (1931)

Largest single: 300m dish Arecibo Obs., Pueto Rico

Since the 1960s: infrared, ultraviolet, x-ray, gamma ray telescopes

Page 29: Fy12 astronomy

Aperture synthesis

Page 30: Fy12 astronomy

Distances to nearby stars: parallaxSecond of arc = 1/3600o ->1pc=3.26 light years

3.3. STARS

Satelite Hipparcos (89) tenfold, 1600ly - 1% diameter of ourgalaxy. Gaia mission (2013), tens thousands ly

Page 31: Fy12 astronomy

Types of spectra- continuous- emission- absorption

Page 32: Fy12 astronomy

• stellar spectra are absorption spectrums• stars are blazing balls of gas: continuous spectrum• some of the colors are absorbed in the atmosphere,

Page 33: Fy12 astronomy

Fraunhofer ,1814, absorption spectrum of the Sun

so far thousands of dark lines, more than 70 elements in the chemical composition of the Sun

Page 34: Fy12 astronomy

Spectral Classes

• U.S. astronomer Annie J. Cannon (1863–1941) examined the spectra of 225,300 stars

• Spectral Classes: OBAFGKMLT

• All visible stars are roughly uniform in composition, made mostly of hydrogen and helium

• differences in the dark line patterns: different surfacetemperatures

Page 35: Fy12 astronomy
Page 36: Fy12 astronomy

One can identify a new star’s spectral class and probable temperature by comparing its spectrum to the images in Figure 3.8

Page 37: Fy12 astronomy

Origin of spectral class characteristics

• At extremely high T, as in O stars, gas atoms are ionized. Only the most tightly bound atoms such as singly ionized helium survive

• Around 5800 K, as in G stars such as our Sun, metal atoms such as iron and nickel remain undisrupted

• Below 3500 K, as in M stars, even molecules such as titanium oxide can exist.

Page 38: Fy12 astronomy

Motions:

Austrian physicist Christian Doppler(1803–1853)Wavelengths are shorter (blueshift) or longer (redshift)when the sourse moves toward or away from us

Page 39: Fy12 astronomy
Page 40: Fy12 astronomy

Other properties:

- Gas density: collisional broadening

- Axial rotation: rotational broadening

- Magnetic field: Zeeman effect

Page 41: Fy12 astronomy

Distinguish a star’s apparent brightness—the way the star appears in the sky—from its luminosity

Propagation: B=L/(4d2)B: (apparent) brightnessL: luminosityd: distance

The Sun’s luminosity is equivalent to 3850 billion trillion 100-watt light bulbs shining all together.

Page 42: Fy12 astronomy

Hertzsprung–Russell diagram

Ejnar Hertzsprung, Danish (1873-1967)Henry Russell, American (1877-1957)

A basic link between luminosities and temperatures

a connection exists between a star’s luminosity and its temperature

Page 43: Fy12 astronomy

-Main sequence:90%, hydrogen fusion-Supergiants, giants:1%, thermonuclearreactions-White dwarfs: 9%,dim, no reactions-Red dwarfs: lowmass stars-Brown dwarfs: substellar, no stable fusion

Page 44: Fy12 astronomy

Spectroscopic distance determination:SpectrumLuminosity class + T + HR diagram Luminosity + apparent brightness + L= 4d2B (propagation) distance d (<3000ly)

Sizes and densities: - Stefan-Boltzmann law L = 4R2T4 - Red giants: very low density compared to the Sun- White dwarfs: 1 spoon – several tons on earth

Page 45: Fy12 astronomy

Mass-Luminosity relation: the more massive a star is, the more luminous it is

Sun, M = 2 × 1030 kg,333,000 times themass of Earth.

Page 46: Fy12 astronomy

3.4 STELLAR EVOLUTION

Birth: - interstellar dust and gas, nebulae, emission nebulae (hot, thin gas 100-1000 solar masses)- protostars form in cold, dark nebulae- gravitational contraction (hydrogen)- temperature and pressure rise greatly- 10 mil. K: nuclear fusion- hydrostatic equilibrium

Orion Nebula, in the constellation Orion

Page 47: Fy12 astronomy

Evolution into main-sequence stars

Page 48: Fy12 astronomy

Why stars shine

• main sequence star: an adult star• very slow evolution• energy source: nuclear fusion reactionshydrogen helium (hydrogen bombs)• E=mc2

• 0.01% of the Sun’s mass changes to sunshine in a billion years

Page 49: Fy12 astronomy

Old age

• A star will shine until all the hydrogen helium

• Our Sun: an average medium-sized star

has been shining for about 5 billion years, should

shine for another 5 billion years

• massive, hot, bright stars die fastest

• Rigel in Orion: only a few million years

red dwarfs are the oldest and most numerous main

sequence stars

Page 50: Fy12 astronomy

Red giants:- core hydrogen fusion ceases- helium core contracts, rising temp.- shell hydrogen fuse faster, luminosity increases- contraction and fusion heat up (100 mil. K)- the star expands to gigantic proportions- surface temperature drops and color turns to red

Page 51: Fy12 astronomy

• Our Sun, like all stars, is expected to change into a

huge red giant when it dies

• That red giant Sun will shine so brightly that rocks

will melt, oceans will evaporate, and life as we know

it on Earth will end

Page 52: Fy12 astronomy

Synthesis of heavier elements

-100 mil. K: nuclear fusion reactionshelium carbon and heavier elements- 2 populations of stars: Pop. I young,metal richPop. II old, metal poor (early universeconsisted almostexclusively of hydrogenand helium)

Page 53: Fy12 astronomy

Variable stars

• Most stars change from red giants to pulsating variable stars before they finally die

• expand and contract and grow bright andfade periodically

• Explanation:- compression ionized helium which is opaque- expansion, cooling, recombination,transparent, contraction

Page 54: Fy12 astronomy

2 properties: - very luminous up to 104 L

- period-luminosity relation distance marker < 10 mil ly

• Cepheid variables: period 1-70 days;example: Polaris, the North Star: every 4 daysdistance marker out to10 mils ly• RR Lyrae variables: less than a day.600,000 ly• Long-period Mira variables, 80-1000 days130 ly away

Page 55: Fy12 astronomy

The deaths of stars: depends crucially on the mass

Low mass stars nebula+coreour Sun will become so big that it will swallow up Mercury, Venus, Earth, and Mars

White dwarfs:• temperature and pressure go up very high• mostly of electrons and nuclei• cooling, crystallized, an immense diamond• gravity 350,000 times that on Earth• turns to dull red, then black dwarf

Page 56: Fy12 astronomy
Page 57: Fy12 astronomy
Page 58: Fy12 astronomy

Exploding stars:High-mass stars supernova

- 8 or more times the Sun’s mass- 600 mil. K, carbon fuse into magnesium- fusion of heavier elements: oxygen, silicon- iron ends these cycles- core compressed, rebounds -> Type II supernova- heaviest elements such as gold and lead areproduced in the explosion our Sun and Earth

Page 59: Fy12 astronomy
Page 60: Fy12 astronomy

Superdense stars: left behind by very massive starsneutron stars, degenerate neutron pressure- more mass than the Sun, 16km across-1 spoon of matter – 100 mils. tons on earth- discovery (1960) - Pulsars: rotating, highly magnetic neutron star (Crab pulsar period 0.0333 sec.)- theory: giant rotating magnet electric generator pair production of elec. + positrons, move alongthe curve field radiation- Superfluidity- Superconductivity- Mass limit 2-3M

Page 61: Fy12 astronomy
Page 62: Fy12 astronomy

Black holes:- Core>3M , another possibility: white dwarfs or neutron stars + companion stars in a bin. system- Schwarzschild radius RS=2GM/c2 ,Sun 3km, Earth 1cm- boundary no light can get out -> event horizon- further shrink -> singularity- Cygnus X1, 1966, binary stars (>20 candidates)- Supermassive black holes at the center of galaxies106-109 M

-3 properties: mass, charge,angular momentum-observation: accretion, gravitational lensing

Page 63: Fy12 astronomy

-Rotating black hole:ring-shaped singularity,accretion disk, wormholes

-Falling into a black hole:an infinite voyage, timedilation, gravitationalredshift, tidal forces

-microscopic black hole: sufficient pressure, LHC-Hawking radiation

Page 64: Fy12 astronomy

3.5 GALAXIES

Milky way: 200 bil. Stars, interstar average distance 5 ly

Page 65: Fy12 astronomy
Page 66: Fy12 astronomy

Our Sun: 250 km/s, 220 mil. years 1 revolution- 25 000 ly from the center- Milky way: 100000 ly across, 10000ly nuclear bulge

Location of stars: star clusters, same age, same origin

Page 67: Fy12 astronomy

Theory check of stellar evolution: all stars leave the main sequence as they age

Below: data, M45 young 70 mil. yearsM3: old, 8 bil. years

Page 68: Fy12 astronomy

Mapping our Galaxy:

• We cannot look > 1000 ly because of dust clouds • spiral structure is mapped by detecting radio waves of 21-cm wavelength, emitted by hydrogen atoms• large, hot gas clouds: continuous radio emission• molecular hydrogen in dark, cool molecular clouds: infrared and ultraviolet wavelengths• gravitation of luminous matter cannot explain observed velocities of stars and gas clouds, gravitational lensing• Our visible Galaxy must contain a lot of dark matter and surrounded by a dark matter halo 300000 ly across

Page 69: Fy12 astronomy

• The nucleus: very massive, compact object ringed by hot, chaotic gas clouds and dust calledSagittarius A* • A massive black hole powers the central gas flows and luminosity

Formation: over 13 billion years ago- 300000 years after Big bang, atoms of hydrogen and helium began to form- Density fluctuations- baryonic matter condense within cold dark matter

Page 70: Fy12 astronomy
Page 71: Fy12 astronomy

Beyond the Milky Way Galaxy:

• Our Galaxy was the only one recognized until 1924• U.S. astronomer Edwin Hubble (1889–1953): proved that some “nebulas” were really galaxies• Large and Magellanic Clouds: companions of ourGalaxy• The Andromeda Galaxy: the closest similar to ours

Page 72: Fy12 astronomy

Classification:

Page 73: Fy12 astronomy

Groupings: - Our galaxy, local group of 40 members- Regular clusters

Active galaxies: central massive object, such as a black hole = mil. Suns

Page 74: Fy12 astronomy

Mysterious quasars: quasi-stellar radio source

- nonstellar spectra, dominated by emission lines- More than 100000 are known- Extraordinary power, thousand normal galaxies- extremely compact, 1 light-day across, not much bigger than our solar system- Ultraluminous centers of distant galaxies- No nearby quasars, the nearest one 800 mil. lyaway- Highest redshift, 90 % of c, ultraviolet light red light on Earth

Page 75: Fy12 astronomy

3.6 THE UNIVERSE

The expanding universe: cosmological redshiftGreatest redshifts: more distant and earlier eras

Cosmological principle: homogeneous and isotropic

Hubble law (1929): v=Hd, H = 23km/sec/mly

Page 76: Fy12 astronomy

H and K of ionized calcium

Page 77: Fy12 astronomy
Page 78: Fy12 astronomy

Standard Big bang theory:- Olber’s paradox- 13.7 bil. years- all matter and radiation were packed together- at 10-43 second: 1032 K- in a few seconds: protons, neutrons, electrons,positrons, neutrino- within minutes: deuterium, helium- 380000 years: cool enough for neutral atoms- several mils. years: stars and galaxies- today: expanding, 74% hydrogen, 24% helium- future: curvature index k (0,-1,1), cosmologicalconstant

Page 79: Fy12 astronomy

k=-1

k=1

Page 80: Fy12 astronomy

Inconstant Hubble constant:- data: deceleration in the past, acceleration now- deceleration if gravity acts alone- dark energy: gravitational repulsion

Matter and energy:- Critical density for a flat universe: 5 hydrogen atoms/m3

- 5% ordinary matter, 23% dark matter 72% dark energy- massive neutrinos, MACHOs massive compact halo objects, WIMPs weakly interacting massive particles mayexist

Page 81: Fy12 astronomy

Cosmic background radiation:- big bang shortwave radiation- now microwave- uniform, isotropic, 2.7K- observed in 1965, Arno Penzias and Robert Wilson

Big bang questions:- Matter - antimatter- horizon problem: disconnected regions have same T- flatness problem: 0=c >50 decimal places- magnetic monopoles inflation 10-38 – 10-32 secWilkinson microwave anisotropic probe (2001-): tinyfluctuations

Page 82: Fy12 astronomy

Shape and size

Page 83: Fy12 astronomy

3.7 THE SUN

Page 84: Fy12 astronomy

Distance:

• The Sun and its planets formed from a rotatingcloud of interstellar gas and dust 5 bil. years ago• The Sun has > 99 percent of the mass

Page 85: Fy12 astronomy

The Sun’s structure

(a) coronararified, hot gas2 mil. K(b) chromosphereglows red, hydrogen gasT 15000K (c) photosphere 5800K(d) convection zone (e) radiation zone (f) core 15 mil K, 200 bil. atm

Page 86: Fy12 astronomy

4 1H 4He + neutrinos + gamma-ray photons

Page 87: Fy12 astronomy

Solar neutrinos

• light provides few clues about the core• 1014 neutrinos/m2/s• exceedingly difficult to detect• R. Davis (1960s), Brookhaven Nat. Lab.100,000 gallons of perchloroethylene (C2Cl4)huge tank deep underground37Cl radioactive 37Ar• Kamiokande, M. Koshiba (1980s), 3000 tons of water, 1100 light detectors, recoiling electron emits light,only a fraction of the expected flux was detectedSuper Kamiokande (1998), neutrino oscillation• Sudbury Neutrino Obs. in Canada (2004): 3 types

Page 88: Fy12 astronomy

Rotation

The period of rotation• at the equator 25 days• slower at middle latitudes• slowest at the poles 35 days

(1999)

Page 89: Fy12 astronomy

Sunspots

• cool blotches on thephotosphere• 4200K• few hours-few months• 2-10 times the Earth• appear in group• most violent activity

Page 90: Fy12 astronomy

• At any one time > 300 sunspotsor none at all - may appear • The number regularly rises and falls ina 11-year cycle• most active with greatest outbursts of energyand radiation for about 4.8 years• 6.2 years solar activity lessens• The current cycle began in 2008

Solar cycle

Page 91: Fy12 astronomy
Page 92: Fy12 astronomy

Magnetism

• Sunspots are like huge magnets• thousands of times > Earth’s magnetic field• measuring Zeeman spectral line-splitting• A weaker magnetic field spreads over the whole Sun• The polarity is reversed every 11 years:22-year solar cycle

Page 93: Fy12 astronomy
Page 94: Fy12 astronomy

Flares and coronal mass ejections

• A flare: tremendous outburst of radiation and material • occur near sunspots• energized by strong magnetic fields

magneticreconnection2106 K

Page 95: Fy12 astronomy
Page 96: Fy12 astronomy

How solar eruptions affect Earth

• as much energy as a billion hydrogen bombs• Gamma-, X-, and ultraviolet-rays in 8.3 minutes.• Flare particles arrive a few hours or days later• These could destroy all life if Earth were not shielded by its magnetic field and atmosphere. • risky for airplane passengers, astronauts, and spacecraft electronics• geomagnetic storms: compasses don’t work• atmospheric storms, satellite damage, surges in electric power and telephone lines, and blackouts.• drag on spacecraft, satellites may plunge.The U.S. space station Skylab (73-79) was a casualty

Page 97: Fy12 astronomy
Page 98: Fy12 astronomy

3.8 THE SOLAR SYSTEM

Page 99: Fy12 astronomy

Origin: solar nebular model

counterclockwise as seen from above, inferior, superiorKuiper (1905-73) belt: icy primordial objects, predicted 1951

Page 100: Fy12 astronomy

Day names

Page 101: Fy12 astronomy

Moon phases

• Full Moon: 12.37 times a year• faint earthshine• synodic month or lunation: 29.5 days

Page 102: Fy12 astronomy

History:-150 A.D. Alexandrian Ptolemy, geocentric model- Polish Copernicus 1543, heliocentric

Page 103: Fy12 astronomy

The phases of Venus

Page 104: Fy12 astronomy

• Galilei (1564-1642), 4 moons orbiting Jupiter-The Church vindicated Galilei in 1992• German Kepler (1571-1630): Kepler’s laws- ellipse, the Sun at one focus- const. area- P2 a3

• Isaac Newton (1642-1727)

Page 105: Fy12 astronomy

Moon’s orbital motion:Sidereal month (one trip around Earth) 27.3 days

Page 106: Fy12 astronomy

Terrestial planets: Mercury, Venus, Earth, MarsJovian planets: Jupiter, Saturn, Uranus, Neptune

Page 107: Fy12 astronomy

3.8 The Planets

Mercury: • fastest, craters • axis of rotation is vertical, no seasons, • very hot 430oC – bitter cold - 180oC,• very thin, unstable atmosphere

Page 108: Fy12 astronomy

Venus: reflecting atmosphere, 97% CO2, temp.480oC (greenhouse effect), pressure 90 atm., dry, rocky

Page 109: Fy12 astronomy
Page 110: Fy12 astronomy

Planet Earth

• crust: lightweight rocks such as granite and basalt• mantle: dense silicate rock• core: molten, metallic layer, probably a solid center• Atmosphere: 78% nitrogen, 21% oxygen, half <6km• Sun’s ultraviolete light produces ozone

(1) Crust 35 km; (2) mantle 2880 km; (3) core 3470 km

Page 111: Fy12 astronomy

Plate tectonics

• 2.5 cm/year• magma convection powers the drift• similar plant and animal fossils

Page 112: Fy12 astronomy

Magnetism: • generated by its rotating liquid iron-nickel core• Reversed at irregular intervals (tens of thousands – hundreds of thousands years)• deflects charged particles from the solar wind

Page 113: Fy12 astronomy

Mars

Page 114: Fy12 astronomy

- There are seasons, -80oC - -5oC- massive volcanoes- the planet’s crust 50 km thick, does not drift- no liquid surface water- ancient catastrophic flooding- water in ice and vapor form- The atmosphere is too thin to block the deadly ultraviolet rays from the Sun,95% CO2

- Perhaps life formed on Mars inthe distant past. Possibly microbes still survive

Page 115: Fy12 astronomy

Jupiter:

- 318 times the mass of Earth- Were it 80 times more massive, nuclear fusion reactions could have started- thick, dynamic, observable atmosphere, mostly hydrogen and helium- Earth-size solid core- Great red spot: a colossal storm observed 300 years- Jupiter’s atmosphere may be similar toEarth’s primitive one

Page 116: Fy12 astronomy
Page 117: Fy12 astronomy

at least 63

Page 118: Fy12 astronomy

Saturn:

• 2nd largest, • 9 rings, consist of billions of dust- to house-size water ice particles• huge multilayered gas ball of mostly hydrogen + < half as much helium• central iron-silicate core surroundedby a metallic hydrogen layer• mass 95 Earth, • volume 844 times• could float in water• 29.5 years to orbit the Sun

Page 119: Fy12 astronomy

Uranus: - discovered with a telescope (1781)- Double the size of the solar system- Mystery till Voyager 2 (1986)- axis of rotation // orbit plane (98o),possibly collision with a planet-size body- Each pole gets 42 years of continuoussun light-atmosphere: 82.5% hydrogen, 15.2% He,2.3% methane (CH4)- No cloud feature (low internal heat)

Page 120: Fy12 astronomy

Neptune: - triumph of theor. astronomy:Uranus did not follow the predicted path. John Adams (1819–1892) in England and Urbain Leverrier (1811–1879) inFrance calculated that its motion was being disturbed by another planet’s gravity.In 1846 Johann Galle (1822–1910) at the Berlin Observatoryin Germany pointed to the predicted spot and found Neptune- great dark spot (1989) giant stormof the size of Earth

Page 121: Fy12 astronomy

Dwarf planets

• Pluto, Eris, Ceres: first dwarf planets in this new category defined in 2006 by the International Astronomical Union• After astronomers saw bigger Eris and other similarKuiper Belt objects, they reclassified Pluto

Page 122: Fy12 astronomy

THE MOON

Synchronous rotation: The Moon rotates on its axis every 27.3 days, the same amount of time it takes to travel around Earth:The same side of the Moon face Earth at all times

Tidal locking

Page 123: Fy12 astronomy
Page 124: Fy12 astronomy

Size and density

• The distance to the Moon: accuracy of a few centimeters by timing how long it takes a laserlight beam to reach reflectors there and return.• diameter of the Moon: 3476 km, ¼ that of earth• average density is 3.34 t/m3, 3⁄5 that of Earth• gravity 1⁄6 that of Earth

Page 125: Fy12 astronomy

Craters

History:-Oldest rocks, 4.3 bil years old-Youngest, from the maria,3.1 bil years old-impact-ejection hypothesis- cooled off 3 bil years ago-Airless, dry, stable surface

Page 126: Fy12 astronomy

max. 7 eclipses/year

Page 127: Fy12 astronomy

COMETS

Important: original material

Page 128: Fy12 astronomy

Tails point away from the Sun(a)Ion tail(b)Dust tail

Origin: Oort cloud (100 bil comets), Kuiper belt

Page 129: Fy12 astronomy

X. LIFE ON OTHER WORLDS

- ability to reproduce and metabolism- Fiery Earth’s earliest atmosphere: amino acids- Hydrothermal vents on the ocean floor- a billion years: RNA and DNA, genetic codes - a common virus is a strand of DNA or RNA- Algae and bacteria fossils in rocks 3 bil. years old

Page 130: Fy12 astronomy

-Multicelled organisms: a billion years ago- first fish: 425 mil years ago- reptiles: 325 mil years ago- dinosaurs: 65 mil years- humans 40000 years

Sun’s habitable zone: between Mars and Venus-Some plants and microbes can survive on Mars

Page 131: Fy12 astronomy

The odds:(1)The number of stars in our galaxy 200 bil(2)The fraction that have planets(3)The average number of planets suitable for life(4)The fraction of life starts that -> intelligent organisms(5)The fraction of int. species that have attempted comm.(6)Guess average lifetime of a civilization-> 1 (ours) to a million civilizations

Page 132: Fy12 astronomy

Extrasolar planetary systems:Circumstellar disks: thick-> planets forming, thin -> already formed

1. Astrometry: tiny wobble in the path of the star2. Spectroscopy: periodic Doppler shifts. First reportedin 1995, many more followed3. Photometry: light output, first reported in 19994. Gravitational microlensing

Page 133: Fy12 astronomy

Star probes:- Pioneer 10: Juiter 1973, beyound Neptune’s orbit 83- Pioneer 11 followed in 1990- Voyagers 1 and 2 are now at the edge of our solar system,should return data till 2020-One coded message was radioed (1974) -> M13 in the constellation Hercules 24000ly away, answer 48000 years-Search for extraterrestrial intelligence (SETI)

2 strategies:-All-sky survey- targeted search, 100ly ofEarth 1-3000MHz

Page 134: Fy12 astronomy

Inventions from outer space:-Smoke detectors-Laser-eye surgery-Magnetic-resonance imaging-Exercise machines-Search and rescue technology-Satellite imagery-Computer enhanced imaging-Plants that purify sewage-Electrolytic water filter-Silicon ribbing for racing swimsuits, Speedos-Supercomputers-Ergonomic chairs for the elderly-Clean labs.

Page 135: Fy12 astronomy

Space isn’t remote at all. It’s only an hour’s drive away if your car could go straight

upwards. Sir Fred Hoyle, in the London Observer, 1979

Reference:Dinah L. Moché ‘Astronomy, a self-teaching guide’

7th edition, John Wiley & Sons, 2009

End of Chapter