You're racing like a fireball Dancing like a ghost You're a gemini and I don't know which one I like the most Deep Purple, Fireball

Assignments: Read: Chapter 29, sections 1-4 (pp. 468-475) Chapter 30, sections 1-7 (pp. 482-496) Problem Set #4 due Thursday 5pm

In Class: Once H-fusing runs out in the core Helium core + H-fusion in a shell around the core - since it's in a shell around the core, it doesn't help support the core -- core continues to shrink -- heats up even more - accelerates the H-shell fusion - star "overheats" - outward pressures no longer balanced by gravity --> star expands A LOT -- becomes a GIANT --> even though the core is really hot, the outer layers actually cool because of the expansion -- surface temperature drops to ~3000 K -- becomes RED --> RED GIANT STAGE while the outer layers are cooling down a bit, the core is still heating up and contracting - nothing to stop it (yet) - eventually the core gets to 100 million K (10x as hot as normal) - and another energy source comes to save the day: He He-fusion is much harder to do than H-fusion He's have 2 p+'s and so repel each other more strongly than H's need a lot of speed to collide He's close enough for fusion --> need really high temperature -- 100 million K Basically a straightforward process: 2He4 + 2He4 --> 4Be8 berylium-8 is really unstable; won't be around for long need to hit it with another He soon also why you need high T and density to He-fuse 4Be8 + 2He4 --> 6C12 net result: 3 He's make a C - got lots of He from the last fusion stage - doesn't get you quite as much energy as the PP process - lower grade fuel; lower efficiency - still, it is a major energy source - and importantly, can help support the core - "Helium flash" - stabilizes core - halts collapse of core AND expansion of outer layers - outer layers contract - the star is now stable again - He-fusing (instad of H-fusing) - making carbon core - core temp = 100 million K (instead of 10 million) - plus an H-fusion shell - sometimes called "Helium Main Sequence" - He isn't as rich a fuel as H - doesn't give you as much energy per reaction - star uses up its supply faster - He MS = 100 million years (H MS = 10 billion years) - then the problem occurs all over again - run out of He - core can't support itself - starts to shrink and heat up - create He-fusing shell around core - so now we have - carbon core: hot, shrinking, not fusing - He shell fusing - H shell fusing - star gets really unstable - starts to pulsate - overheats from shell fusing - swells up - overcools from swelling up - fusion slows - star contracts - overheats He and H shells - too much fusion - swells up - etc. - pulsations get really wild - pieces of the outer parts of the star - literally shaken off star - blown away - in about 100 years, all but the core has been blown off the star - result: -- really small, hot core exposed -- shell of outer layer star bits thrown off into space -- PLANETARY NEBULA (terrrible name) Planetary nebulae - shells of gas around central star - shells are heated by central star - glow in spectral line emission (this stuff has low density) - shells represent replenishment of ism - stars are made from junk in the ism - low mass stars return some of their stuff --> recycling - surface of central star is very hot (100,000 K) - most of it's continuous light is emitted in the ultraviolet - no fusion to hold it up - continues to contract under the force of gravity - a whole solar mass might contrat to smaller than the Earth - what stops it from contracting to a point? - electron degeneracy - electrons don't like each other - same charges; repel - if you push them too close together - they push back - create a pressure - not thermal pressure (created by motions of particles) - at high densities, matter becomes nearly incompressible - ie., pushes back _hard_ - can stabilize against gravity White dwarfs stars - support does not depend on temperature - white dwarf just sits there - slowly cooling down - gets fainter and fainter - eventually undetectable - low mass stars die as cooling hulks of ash --------------------- Why is this the fate of only low-mass stars? - why doesn't this happen to bigger stars, and what happens instead? Life as a massive star is very different - more massive - dominates the evolution - central temperatures have to be higher - to support against extra gravity - "big appetite" - that means fast H-burning on the Main Sequence - this is why massive MS stars are so much more luminous than low mass MS stars - even though they have a lot more mass - they burn through it A LOT more quickly - MS lives as short as 10 MYR (instead of Sun's 10 BYR) - generate a substantial He core - goes into RED GIANT stage - H shell burning - He flash and He core burning - makes C core --> mostly the same as Low mass stars, only faster Bigger stars make bigger carbon cores more mass ->> more gravity --> more squishing --> more temperature - just as raising the temperature in a red giant got us He fusing - so too, raising the temperature even more gets us C fusing - C core shrinks and heats to a few billion K - SUPERGIANTS - sizes as big as the orbit of Jupiter (5x earth's orbit) - very red, but outrageously luminous - L = 1 million suns C fusion goes very quickly and doesn't produce much energy - that's why it has to burn so quickly - makes lots of heavy elements - probably the only place where heavy elements formed - if the universe started as only H this is the _only_ place to make heavy elements - Ne, Si, Mg, C fusion only stays the pull of gravity for a little while maybe thousands of years - then the core starts to contract and heat up again - when even hotter, Ne and Si can fuse - process continues until you get to iron (Fe) Fe fusion doesn't get you any energy - it's the world's most stable nucleus - adding more stuff to it only makes it less stable - costs energy - remember how splitting U gives you energy? Iron is the end of the line - no matter how hot it gets the core, no more fusion - ie., no more thermal support