Fy12 astronomy
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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
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
• 100000ly across• Solar system:790000km/h220mil years – onetrip
The solar system in the Milky Way galaxy
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
1, 87 ,
cos
18 20
correct value 390
oS
L
S
LS
L
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’
Location on Earth
• latitude lines• longitude lines(meridian lines)
Celestial sphere
Constelations
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
(a) autumnal equinox ; (b) summer solstice ; (c) vernal equinox ; (d) winter solstice
Equinoxes and solstices
Tides
3.2 Light and Telescopes
What is light?
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
Astronomical observations: 2 atmospheric windows- optical/visible light- some infrared and radio windows dark sites, dry, thin, steady air
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
Reflecting telescopes:
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
Useful magnification
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
planned for 2020
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
Radio astronomy (1931)
Largest single: 300m dish Arecibo Obs., Pueto Rico
Since the 1960s: infrared, ultraviolet, x-ray, gamma ray telescopes
Aperture synthesis
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
Types of spectra- continuous- emission- absorption
• stellar spectra are absorption spectrums• stars are blazing balls of gas: continuous spectrum• some of the colors are absorbed in the atmosphere,
Fraunhofer ,1814, absorption spectrum of the Sun
so far thousands of dark lines, more than 70 elements in the chemical composition of the Sun
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
One can identify a new star’s spectral class and probable temperature by comparing its spectrum to the images in Figure 3.8
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.
Motions:
Austrian physicist Christian Doppler(1803–1853)Wavelengths are shorter (blueshift) or longer (redshift)when the sourse moves toward or away from us
Other properties:
- Gas density: collisional broadening
- Axial rotation: rotational broadening
- Magnetic field: Zeeman effect
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.
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
-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
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
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.
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
Evolution into main-sequence stars
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
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
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
• 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
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)
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
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
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
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
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
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
-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
3.5 GALAXIES
Milky way: 200 bil. Stars, interstar average distance 5 ly
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
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
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
• 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
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
Classification:
Groupings: - Our galaxy, local group of 40 members- Regular clusters
Active galaxies: central massive object, such as a black hole = mil. Suns
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
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
H and K of ionized calcium
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
k=-1
k=1
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
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
Shape and size
3.7 THE SUN
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
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
4 1H 4He + neutrinos + gamma-ray photons
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
Rotation
The period of rotation• at the equator 25 days• slower at middle latitudes• slowest at the poles 35 days
(1999)
Sunspots
• cool blotches on thephotosphere• 4200K• few hours-few months• 2-10 times the Earth• appear in group• most violent activity
• 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
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
Flares and coronal mass ejections
• A flare: tremendous outburst of radiation and material • occur near sunspots• energized by strong magnetic fields
magneticreconnection2106 K
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
3.8 THE SOLAR SYSTEM
Origin: solar nebular model
counterclockwise as seen from above, inferior, superiorKuiper (1905-73) belt: icy primordial objects, predicted 1951
Day names
Moon phases
• Full Moon: 12.37 times a year• faint earthshine• synodic month or lunation: 29.5 days
History:-150 A.D. Alexandrian Ptolemy, geocentric model- Polish Copernicus 1543, heliocentric
The phases of Venus
• 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)
Moon’s orbital motion:Sidereal month (one trip around Earth) 27.3 days
Terrestial planets: Mercury, Venus, Earth, MarsJovian planets: Jupiter, Saturn, Uranus, Neptune
3.8 The Planets
Mercury: • fastest, craters • axis of rotation is vertical, no seasons, • very hot 430oC – bitter cold - 180oC,• very thin, unstable atmosphere
Venus: reflecting atmosphere, 97% CO2, temp.480oC (greenhouse effect), pressure 90 atm., dry, rocky
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
Plate tectonics
• 2.5 cm/year• magma convection powers the drift• similar plant and animal fossils
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
Mars
- 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
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
at least 63
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
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)
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
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
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
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
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
max. 7 eclipses/year
COMETS
Important: original material
Tails point away from the Sun(a)Ion tail(b)Dust tail
Origin: Oort cloud (100 bil comets), Kuiper belt
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
-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
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
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
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
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.
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