ASTR3 Week 8 Notes - Week 8-Lecture 21 Stellar Evolution...

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Week 8 11/16/15 ---Lecture 21: Stellar Evolution Chapter 12 - the Evolution of Low-mass stars Expansion into a Giant o When H in core exhausted, no internal energy source → He core contracts & heats up o H-fusion begins in a shell surrounding He core o Outer layers of star absorbs the energy, the expand and cool Star becomes first a subgiant and then a Red Giant 10 to 100 times radius of present sun o Timescale: < 10 9 years Helium Flash, part I o As He core contracts, densities become extremely high o Densities so high that core doesn’t behave like regular gas (where pressure is proportional to density X temperature). Gas is supported by degeneracy pressure o Pressure comes from trying to squeeze lots of electrons into small volume What is a Degenerate Electron Gas? o If you squeeze matter so much that the volume of a sugar cube has a ton of mass, then matter can no longer take its familiar form as atoms, ions, and molecules o The electrons are then free to roam, and this electron gas requires quantum mechanics for its description
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o In this gas, most electrons must have very high speeds, so they exert a very high pressure electron degeneracy pressure o Unlike a thermal gas, this pressure has nothing to do with the temperature of the gas → result of compression and quantum mechanics, not how hot the gas is Helium Flash, part I o As more He “ash” from H-burning shell collects, He core also becomes hotter o When core temperature reaches 10 8 K, fusion of……. o /……….. Helium Flash o The stellar thermostat discussed previously is broken in low-mass red giant stars because degeneracy pressure supports core instead of normal heat pressure o Core temperature rises very rapidly when helium fusion begins - helium “flash” - ……… o ……………. o Do we see helium flash? No, it’s in the star After Helium Flash: o The star re-establishes equilibrium, but now it’s smaller and hotter o And now it’s burning Helium in its core o → “horizontal branch” A second main-sequence → Burning helium to form carbon In main sequence → burning hydrogen to form helium Here we go again: o When core helium is exhausted, star has an inert carbon core
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o What does that carbon core do? It contracts! o And again , this pours energy into the surrounding stellar envelope o So the star expands again o → Becomes an even brighter red giant, an “asymptotic giant branch” or AGB star AGB Star o An AGB star is so cool that dust forms in its atmosphere o And so bright that the pressure of the radiation blows out the dust, creating a wind. Dust takes gas with it!
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