Stars and Galaxies 1
Characteristics of Stars 2
Star - body of gases that gives off great amounts of radiant energy as light and heat 3
Most stars look white but are actually different colors Antares - red Rigel - blue-white Arcturus - orange 4
Stars vary in size and color Some only 20 km diameter Sun (medium size) diameter is 1,392,000 km 5
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Astronomers (scientists who study stars and space) used to use telescopes on Earth Now, they use the Hubble Space Telescope Seeing Stars 7
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Interference from atmosphere makes viewing stars difficult Astronomers use starlight to analyze Put starlight through spectrometer - separates light into different colors (spectrum) 9
Dark-Line Spectra Especially useful to scientists Reveals characteristics about stars composition, temperature Every element has characteristic spectrum Colors and lines in spectrum of star tell which elements star is made of 10
Hydrogen and helium make up most stars 11
Temperature Temperature indicated by color Color Surface Temperature ( C) Blue Above 30,000 Blue-white 10,000-30,000 Blue-white 7,500-10,000 Yellow-white 6,000-7,500 Yellow 5,000-6,000 Orange 3,500-5,000 Red Less than 3,500 12
Motion Two kinds of motion associated with stars Actual motion Apparent motion 13
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Apparent Motion Because of rotation of Earth on axis, stars seem to move across night sky, but it s actually just the Earth s movement 15
Notice they seem to move around 1 center point? Polaris star directly above North Pole Earth rotates around this point, so Polaris does not appear to move 16
Earth s revolution around sun causes stars to appear to move in a second way Stars located on side of sun opposite Earth are blocked by the sun 17
Different stars visible during different seasons, depending on where Earth is in relation to the sun 18
Some stars are always visible b/c they are close to Polaris Circumpolar stars - stars circling close to Polaris 19
Stars of Little Dipper are circumpolar 20
Most stars have 3 motions: Rotate on axis May revolve around another star Move away from or toward the Earth 21
Spectrum of star moving toward or away from Earth appears to shift Doppler effect - apparent shift in wavelength of light produced by light source moving toward or away from observer 22
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Moving toward the Earth wavelength decreases, more toward blue end of spectrum (blue shift) Moving away from Earth wavelength increases, more toward red end of spectrum (red shift) 24
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Most distant galaxies have red-shifted spectra Galaxies are moving away from Earth 26
Distance to Stars b/c distances are SO FAR, measured in light years - distance light travels in one year Speed of light = 300,000 km/s 27
Light travels about 9.5 trillion km in 1 year Light from sun takes 8 minutes to reach Earth 28
Closest star in this system is Proxima Centauri (4.2-light years away) 29
Brightest star seen from Earth is Sirius ( 9 light years-away) 30
Polaris (North Star) is 700 light-years away 31
Determining Distance One way astronomers use to determine distances is parallax As earth circles sun, observers study stars from slightly different angles 32
During 6-month period, closer star shifts relative to stars farther away Closer star, more shift 33
Example of Parallax Hold out your arm in front of you with your thumb up Close one eye and note the position of your thumb against the background Open that eye and close the other one Does your thumb shift? The closer your thumb to your face, the greater the shift 34
Star Brightness Astronomers estimate brightness of far away stars by looking at spectrum Compare estimate of true brightness with apparent brightness From these measurements, astronomers can calculate the distance from Earth35
Stellar Magnitudes Over 3 billion stars can be seen through telescopes Only 6,000 of these visible to eye From earth-orbiting telescopes, over 1 trillion stars can be seen 36
Visibility depends on brightness and distance from Earth Astronomers use 2 scales to describe brightness Apparent magnitude Absolute magnitude 37
Apparent Magnitude Apparent magnitude - brightness of a star as it appears from earth Use special instrument to measure Measurement assigned a number on a scale Dimmer stars = higher numbers 38
Most powerful telescopes can detect stars with apparent magnitude of +29 Faintest star seen by eye has apparent magnitude of +6 This is called a 6th magnitude star 39
Absolute Magnitude Absolute magnitude - true brightness How bright the star would appear from 32.6 light-years away If sun was 32.6 light-years away, it would be 5th magnitude So absolute magnitude of sun = +5 Most stars are between -5 and +15 40
Every star has apparent and absolute magnitude values Relationship between two depends on distance between earth and the star Stars that are less than 32.6 ly appear brighter These stars have apparent magnitudes lower than absolute magnitudes 41
Sun is only a fraction of 1 ly from earth Apparent magnitude = -26.8 Absolute magnitude = +5 42
Classification of Stars Plotting surface temperatures against absolute magnitude reveals a pattern H-R diagram Brightness increases as surface temperature increases 43
Most stars fall in band running through middle Main-sequence stars Upper right corner are cool, bright stars - giants Some so big they are called supergiants 44
Lower left of H-R diagram are hot but dim (very small) White dwarf - usually about the size of Earth 45
Stellar Evolution SECTION 2 46
Typical star exists for billions of years Astronomers never able to observe one star through its whole life Instead, they develop theories about evolution of stars by studying stars in different stages 47
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Nebula Star begins in a nebula - cloud of gas and dust Made of about 70% H, 28% He, 2% other 49
Gravitational force in nebula is weak When force hits nebula (close star explodes, another nebula hits it), it starts to contract 50
According to Newton, distance, gravity As density of particles, gravity between them Particles come together area of denser matter builds up 51
Gravity causes dense areas to shrink Areas become smaller, start to spin faster Like ice skater pull arms into body, spin faster 52
Shrinking, spinning area flattens into protostar 53
Protostar Gravity pulls more material toward center of protostar Pressure, heat 54
Protostar contracts and heats up for millions of years Eventually gas is so hot it becomes plasma - 4th state of matter where electrons ripped from atoms of gas and move freely Temperatures increase until about 10,000,000 C At this temp, nuclear fusion begins 55
Nuclear Fusion Less massive nuclei (hydrogen) combine to form more massive nuclei (helium) Releases huge amounts of energy Once started, continues for billions of years 56
Nebula can make more than 1 star Sometimes two or more made (binary stars) Revolve around each other 57
Nebula can also make planets that revolve around star Our solar system from the same nebula 58
Main-Sequence Stars Second and longest stage in life of star Energy made in core of star as H atoms fuse into He atoms 59
How Much Energy? When 1 g hydrogen converted to helium, the energy released is enough to keep a 100-W light bulb burning for 3,000 years 60
Energy moves outward (from core of star) in the same way energy rises up through boiling water Star does not expand (gravity keeps it in) Star maintains stable size as long as it has enough H to fuse to He 61
Giants and Supergiants 3rd stage when almost all H is fused to He Without H as fuel, star contracts under gravity Contraction increases temperature in core Higher temperature fuses He into C H fusion continues in shell around helium core 62
Combine H fusion (in outer shell) and He fusion (in core) releases energy Causes outer shell to expand greatly Outer shell of gases cools Becomes red giant/red supergiant 63
Red giant - 10+ times bigger than sun Red supergiant - 100+ times bigger than sun 64
Stages in life of star cover ENORMOUS periods of time Scientists estimate over 5 BILLION YEARS, the sun (main-sequence) has only fused 5% of its hydrogen 65
White Dwarf Stars End of giant stage is the end of helium fusion Energy no longer available Star loses outer gases Exposes a core Becomes planetary nebula 66
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Gravity causes last matter to be pulled in What s left - hot, dense core of matter (white dwarf) Shine for billions of years before cooling completely When ALL energy gone - black dwarf Don t exist yet, universe not old enough 69
Novas Some white dwarfs do not cool and die During cooling, large explosions occur that release energy, gas and dust Nova - large explosion from white dwarf One white dwarf may become nova several times 70
Astronomers think nova happens in white dwarf near main-sequence or giant star White dwarf is denser Greater gravity than nearby star Gases from nearby star build up on white dwarf Pressure builds until explosion 71
explosion that it blows itself apart 72 Supernova Stars with masses 10-100 times greater than sun Explosions are 100 times brighter than novas 1054 Chinese saw explosion so bright it was seen during the day for 3 weeks Supernova - star that has such large
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After supergiant stage, stars contract with greater gravitational force than smaller stars Collapse produces such high pressure and temperature that fusion starts again Carbon atoms fuse into magnesium, then to iron 75
Continues until core is almost all iron Fusion stops Iron absorbs energy from gravity Core collapses, causing outer part of star to explode Explosion releases same amount of energy an ordinary star produces in its ENTIRE LIFE 76
Neutron Stars After explosion, nova may contract into small but DENSE ball of neutrons - neutron star Spoonful of neutron star would weigh 100 million tons on earth Rotate quickly Diameter about 30 km 77
Pulsars Some neutron stars release 2 beams of radiation Sweep across universe like lighthouse Called pulsars Radiation detected as radio waves 78
Black Holes Some massive stars make extras too massive to become neutron stars Contraction crushes dense core Leaves hole in space - black hole Gravity so great not even light can escape 79
Black holes don t give off light so locating them is difficult Find by effects on nearby stars Matter from nearby star pulled into black hole Just before it goes in, X rays are released 80
Star Groups SECTION 3 81
When you look into night sky, looks like individual stars Only 1 in 4 stars is actually a single star About 1/3 are double stars Rest are 3 or more stars 82
Constellations Constellation - pattern of stars Astronomers recognize 88 constellations 83
Some named for real or imaginary animals Ex. Ursa Major the great bear Or Draco, the dragon 84
Others named for ancient gods or legendary heros Ex. Hercules Or Orion, the Hunter 85
Astronomers divided sky into sections using constellations Can use like map to locate specific star Label stars in each constellation according to apparent magnitude Brightest star in constellation labeled alpha, α second brightest labeled beta, β 86
Galaxies Galaxy - large-scale group of stars Held together by gravity Usually about 100,000 light years in diameter Average contains about 1 billion stars 87
Galaxies also contain gas and dust clouds (nebulae) There are bright and dark nebulae Bright glow from hot gases inside OR reflecting light from nearby stars Dark absorb light of stars behind them 88
The Horsehead Nebula is a dark nebula. 89
Astronomers estimate between 50 billion and 1 trillion galaxies in known part of universe Large Magellanic Cloud and Small Megallanic Cloud are closest to earth s galaxy (Milky Way) Still 150,000 light years away 90
Within 3 million light years are about 17 other galaxies These plus Milky Way are called the Local Group 91
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Types of Galaxies Classified into 3 main types Spiral has nucleus (center) of bright stars Flat arms that spiral around center Arms have millions of young stars, gas, dust Barred spiral galaxies - have bar of stars through center 93
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2. Elliptical Galaxies Very bright in center No spiral arms Have no young stars Contain very little gas and dust 95
3. Irregular Galaxies No particular shape Usually smaller and fainter Probably so small not enough gravity to pull together into a shape 96
Look at night sky you see band of stars across the sky Called Milky Way because of milky appearance It is actually the disk of Milky Way Galaxy Milky Way Galaxy 97
Diameter = 100,000 light years Nucleus = 2,000 light years thick Sun is about 30,000 light years from center Milky Way Galaxy rotates 98
Sun is on one arm Moves around center at 250 km/s 99
Completes 1 revolution around center of galaxy every 200 million years 100
Star Clusters Milky Way contains single stars and star clusters Groups of hundreds of stars May be open clusters or globular clusters 101
Open Globular More loosely shaped Less stars Located on arms of disk Has spherical shape More stars Located in central core of galaxy 102
Binary Stars Binary stars - pairs of stars that revolve around each other Most stars in galaxy are binary or multiple-star systems 103
Formation of Universe Big bang theory - all matter and energy in universe was concentrated in small volume 12-15 billion years ago, big bang happened Universe started expanding Still moving outward 104
1960 astronomers discovered objects in universe 12 billion light years from earth Light we see is 12 billion years old Quasars - star-like objects that give off radio waves and X rays Among the first objects to be formed from big bang 105
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