Stars and the Sun• a star is a ball of hot plasma

(like a gas with an electrical charge)

• 90% of the matter in the universe is plasma

• it is not “on fire” • it does not “burn” like a fire

does

Why is a star hot?• Nuclear fusion

– 4 hydrogen atoms are pushed together to make 1 helium atom

– left over particles released as energy

– the opposite of what happens in nuclear power plants (fission) where atoms are split up

Anatomy of a star• Has layers just as

the Earth does• Core: center where

fusion takes place (15 mil

K)• Radiative zone:

energy moves out from center

• Convective zone: energy rises and sinks, some escapes

Anatomy of a star• Photosphere: light

we see comes from here

• Has atmosphere just as the Earth does

• Chromosphere: first layer of

atmosphere … is pink!

• Corona: extends out into space

Brightness of a star• depends on:

– how big the star is (bigger = brighter)– how far from Earth it is (closer = brighter)– how hot it is (hotter = brighter)

• called the magnitude (2 kinds)• as seen from Earth is called the apparent

magnitude

Brightness of a star

• Kinds of brightness:– Luminosity: measure of the amount of

energy given off. Measured as compared to the Sun (1), logarithmic 102, 104

– Apparent magnitude: as seen from Earth, lower (including negative) is

brighter!– Absolute magnitude: if all stars were

same distance from Earth, lower (including negative) is brighter!

Spectrum

• the visible light waves that a star gives off or absorbs

• Different for each element…like a fingerprint

• can tell us:– what star is made of how fast moving– how far away it is how fast spinning– if its moving to/away

Spectrum

Star color• Related to temperature• Stars give off all wavelengths of

light, but usually more of one wavelength (color)

• Red = ~ 3,500K Orange = ~ 5,000K

• Yellow = ~ 6,000K White = ~ 10,000K• Blue = ~ 15,000K +

HR Diagram• Plots brightness and temperature

HR Diagram• Groups:

– Main sequence: diagonal line, 90% of stars– White dwarfs: hot, but small, lower left– Red giants/supergiants: cool, but big,

upper right– Blue Giants: hot and big, upper left

• Grouped by spectrum:– O B A F G K M– Oh, Be A Fine Girl (Guy) Kiss Me

Stars Life Cycle

• Goes through different stages, depending on mass: more mass = bigger and hotter

• All stars start as a cloud of gas and dust (nebula)

• Condenses and eventually fusion starts, but very cold star (protostar) and usually can’t be seen in nebula

Nebulae and protostars

Stars like the Sun

• Become Main Sequence stars next as gas and dust continue to condense and the nebula disappears

– Fuse H He for about 10 billion years

• Runs out of H, core shrinks, outer layers cool and expand (red giant)– Big enough to swallow first 3 planets– Uses He other elements for about 10

million years

Stars like the Sun

• Runs out of He, core shrinks, outer layers float into space (planetary nebula)

• Core left over, small and hot (white dwarf)

• Eventually fuses up to carbon, ends as small cold ball of carbon (black dwarf)

Stars like the Sun

Stars bigger than the Sun

• 8x or more massive• Nebula protostar• A star with much more mass than the

Sun will be bigger and brighter (blue giant) but not last as long

• Then it will get bigger and cooler (red supergaint)

• It can fuse elements up to Iron (Fe #26)

Stars bigger than the Sun

• When core is made of iron, it can’t condense anymore. The great gravity

pulls the outer layers in quickly and they hit the iron core and rebound,

exploding in a supernova• In that explosion elements heavier

than iron are formed

Stars bigger than the Sun

• The core is usually left over. – If the core isless than 3 times the mass of

the Sun, it will become a neutron star … something so dense that electrons are pushed into the nucleus and cancel out protons and make all neutrons! Some give out regular pulses of radio waves (pulsar).

– If the core is 3x or more massive than the Sun it collapses again into a black

hole…so dense that not even light escapes!

Stars bigger than the Sun

Relative star sizes

Relative star sizes

Galaxies

• Collections of millions or billions ofstars

• Also interstellar matter … gas and dust

• Grouped together in clusters ours is called the Local Group

• Clusters form superclusters and so on

Galaxies• 3 basic shapes:

– Spiral• Central bulge with spiral arms• Older stars towards center, younger ones on

outside• Sometimes a bar instead of a bulge … barred

spiral

– Elliptical• Oval or round, no arms• Mostly older star , little interstellar matter

– Irregular• No regular shape• No pattern to kinds of stars or amount of

interstellar matter

Spiral Galaxies

Elliptical Galaxies

Irregular Galaxies

Galaxies• Milky Way

― Most likely a spiral galaxy…strip of stars is the spiral arm

― Hard to tell without leaving it

• Quasar― Most distance and radiant object i

space― Very young galaxy

• Change over time … as stars change and run into each other

Colliding Galaxies