Post MS Evolution

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Post - MS Evolution
How do stars die? It depends upon their mass
1 M? Post - MS Evolution What will happen to the
Sun?
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Look in the Core
Initial Composition 70 H 27 He
H? He
After 5 Billion years of fusion Core
Composition 65 He 35 H
He
What happens when H runs out?
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End of MS Phase
Core - hot, helium rich, compressing Energy
production is down - shell burning Very hot core
- Puffs up star Core compression - hotter,
denser Core becomes degenerate - cant be crushed
any more Temperature increases
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Will this ever end? Yes, when the Core
temperature 100 Million K Helium Fusion
Starts! Catastrophic start - Helium Flash Helium
Fusion - Triple alpha He ?C, O
He ?C, O
H?He
C, O
Next? Depends on Mass!
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Summary of Evolution
Sun on ZAMS 5 billion years ago
Sun now
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End of MS life 5 billion years from now Core
energy production - H burning shell Outer layers
start expanding
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Sun enters Red Giant Phase Core - degenerate,
hotter Outer layers expanding more
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Helium Flash Core no longer degenerate Helium
fusion starts
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Steady Helium Fusion C, O core growing
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Helium runs out Degeneracy returns Uh-oh! What
next? Larger mass - more fusion Smaller mass -
end of the road
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No more major fusion Core gets hotter,
degenerate Outer layers expand and expand and
expand Winds, helium shell flashes, thermal
energy push outer layers out and away Major mass
loss event - outer layers blown away! Star enters
into its Planetary Nebula Stage
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Planetary Nebula Stage
Bipolar
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What are White Dwarfs like?
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White Dwarfs
Hot - Temperature typically 10,000s K Small -
Radius Earths Radius Faint Dense - 1,000,000
gm/cc 1 Volkswagen/cc
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Mass - limited to less than 1.4 M ? - why?
Subramanyan Chandrasekhar
The Chandrasekhar Limit 1.4 M?
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White Dwarf in a close binary system..
Nova - Close binary white dwarf system 100,000
L? Bright for days or weeks Has the potential to
repeat
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Evolutionary scenarios - Very low mass stars (M
lt 0.1 M ?) Main Sequence (H fusion) Red
Giant Planetary Nebula White dwarf (made of
helium) Solar mass stars Main Sequence (H
fusion) Red Giant (He fusion) Planetary
Nebula White dwarf (made of C, O) More massive
(2 M? lt M lt 8 M ?) Main Sequence (H
fusion) Red Giant (He, C, O,
fusion) Planetary Nebula White dwarf (made of
C, O, Ne, Mg, Si, etc.)