Stars and Galaxies

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Characteristics of Stars

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• Star - body of gases that gives off great amounts of radiant energy as light and heat

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•Most stars look white but are actually different colors

• Antares - red•Rigel - blue-white• Arcturus - orange

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• Stars vary in size and color• Some only 20 km diameter• Sun (medium size) diameter is 1,392,000

km

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Seeing Stars• Astronomers (scientists

who study stars and space) used to use telescopes on Earth• Now, they use the

Hubble Space Telescope

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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)

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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 of10

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Hydrogen and helium make up most stars

Temperature• Temperature indicated by color

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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

Motion• Two kinds of motion associated with stars• Actual motion• Apparent motion

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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

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• 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

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• 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

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Different stars visible during different seasons, depending on where Earth is in relation to the sun

• Some stars are always visible b/c they are close to Polaris•Circumpolar stars - stars circling close to

Polaris

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• Stars of Little Dipper are circumpolar

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•Most stars have 3 motions:•Rotate on axis•May revolve around another star•Move away from or toward the Earth

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• 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

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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)

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•Most distant galaxies have red-shifted spectra•Galaxies are moving away from Earth

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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

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• Light travels about 9.5 trillion km in 1 year

• Light from sun takes 8 minutes to reach Earth

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•Closest star in this system is Proxima Centauri (4.2-light years away)

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• Brightest star seen from Earth is Sirius ( 9 light years-away)

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Polaris (North Star) is 700 light-years away

Determining Distance•One way astronomers use to determine

distances is parallax• As earth circles sun, observers study

stars from slightly different angles

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During 6-month period, closer star shifts relative to stars farther awayCloser star, more shift

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

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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 numbers38

• 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 star39

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 magnitudes41

• Sun is only a fraction of 1 ly from earth• Apparent magnitude = -26.8 • Absolute magnitude = +5

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Classification of Stars• Plotting surface

temperatures against absolute magnitude reveals a pattern• H-R diagram• Brightness increases as

surface temperature increases

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• 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

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• Lower left of H-R diagram are hot but dim (very small)

•White dwarf - usually about the size of Earth

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SECTION 2

Stellar Evolution

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• 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

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Nebula• Star begins in a nebula -

cloud of gas and dust•Made of about 70% H,

28% He, 2% other

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•Gravitational force in nebula is weak•When force hits

nebula (close star explodes, another nebula hits it), it starts to contract

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• According to Newton, distance ↓, gravity ↑• As density of particles ↑, gravity between

them ↑• Particles come together – area of denser

matter builds up

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•Gravity causes dense areas to shrink• Areas become smaller, start to spin faster

• Like ice skater – pull arms into body, spin faster

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Shrinking, spinning area flattens into protostar

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Protostar•Gravity pulls more material toward center

of protostar• Pressure ↑, heat ↑

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• 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

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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

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•Nebula can make more than 1 star• Sometimes two or more made (binary

stars)•Revolve around each other

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•Nebula can also make planets that revolve around star•Our solar system from the same nebula

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Main-Sequence Stars• Second and longest

stage in life of star• Energy made in

core of star as H atoms fuse into He atoms

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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

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• 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

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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

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• 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

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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

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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

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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

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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

explosion that it blows itself apart 72

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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 iron75

•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

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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 waves78

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

escape79

• 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

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SECTION 3

Star Groups

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•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

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Constellations•Constellation - pattern of stars

• Astronomers recognize 88 constellations

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• Some named for real or imaginary animals• Ex. Ursa Major – the great bear

•Or Draco, the dragon84

•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

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•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

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89The Horsehead Nebula is a dark nebula.

• 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

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•Within 3 million light years are about 17 other galaxies• These plus Milky Way are called the

Local Group

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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

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3. Irregular Galaxies•No particular shape•Usually smaller and fainter• Probably so small not enough gravity to

pull together into a shape

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Milky Way Galaxy• 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 Galaxy97

•Diameter = 100,000 light years•Nucleus = 2,000 light years thick

• Sun is about 30,000 light years from center

•Milky Way Galaxy rotates98

• Sun is on one arm•Moves around center at 250 km/s

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•Completes 1 revolution around center of galaxy every 200 million years

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Star Clusters•Milky Way contains single stars and star

clusters

•Groups of hundreds of stars

•May be open clusters or globular clusters101

Open Globular

• More loosely shaped • Less stars• Located on arms of disk

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Has spherical shapeMore starsLocated in central core of galaxy

Binary Stars• Binary stars - pairs of stars that

revolve around each other

• Most stars in galaxy are binary or multiple-star systems

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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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