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 10 2, 10 4 –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 ofhow fast moving –how far away it ishow 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 = ~ 3,500KOrange = ~ 5,000K Yellow = ~ 6,000KWhite = ~ 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

Galaxies Collections of millions or billions of stars 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