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Lecture 24

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PHYS 183
Tracy Webb

Lecture 24: March 13 , 2013  review: when He runs out the star collapses again & creates a He burning shell around an inert C & O core  causes the blow off of the outer layers creating a planetary nebula  remember that planetary nebulas are 3D, circular objects  evolution to a white dwarf o white dwarfs maintain gravitational equilibrium o there is a He core that has been converting He to O that collapses since it can’t support itself o the star will continue to collapse until it has a high enough temperature in the core that it can fuse C & O, but the temperatures never get this high (this is due to electron degeneracy pressure) o in a low mass star, the EDP kicks in earlier o the star becomes stable again (not collapsing) & has gotten rid of its outer layers  inert, stable core is exposed o there is no more energy generation though o all outer layers are gone and you just have the core left o white dwarfs are very faint because they are very small ( they are also compact) o they slowly cool over time o they are a very bizarre form of matter  Chandrasekhar Limit o mass-radius relation for white dwarfs: more mass means smaller radius o only works up to 1.4 solar masses o electrons in white dwarfs have to orient themselves in a certain way for this to work since must become more dense to support itself  sun’s life o protostar: 30 million years o main sequence: 10 billion years o Red Giant: 1 billion years o horizontal branch: 100 million years o double shell: 30 million years o planetary nebula: 10 000 years o white dwarf: forever?  mass and fusion: recap o high mass stars are at the top left of the HR diagram; intermediate mass stars in the middle & low mass stars bottom right  hydrogen fusion: the CNO cycle o CNO = carbon, nitrogen, oxygen o end result in the same as the PPP cycle: 4 protons  1 He o heavier elements act as catalysts which are objects or additives that speed up a reaction o very high temperatures are required to overcome repulsive force o this produces much higher luminosities  high mass stars moving off the main sequence o first post main sequences life stages of a high mass star are very similar to that of a low mass star o H shell burning
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