Stellar Death

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Stellar Death

• A Spectacular End!

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Stellar Death

• White Dwarfs

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Life after the Main Sequence
How do stars evolve? It depends upon their mass.
1 M? Stellar 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 Hydrogen runs out?
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Main Sequence Phase Ends
Core is hot, helium rich. Overall energy
production is down - shell burning only, no core
fusion. Very hot core exerts outward radiative
pressure on outer stars envelope (outer
layers). Puffs up star whooshes the outer
layers. Core continues gravitational compression
growing hotter and denser. Core becomes
degenerate - cant be crushed any more. Electron
Degeneracy Pressure.
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Will this ever end? Yes, when the Core
temperature 100 Million K Helium Fusion
Starts! Catastrophic start - Helium
Flash Removes degeneracy in core Initiates
second life for star Helium Fusion - Triple
alpha process He ?C, O
He ?C, O
H?He
C, O
Now what happens? Depends on Mass!
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H.R. Diagram Stellar Evolution
Sun on Z.A.M.S. 5 billion years ago
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H.R. Diagram Stellar Evolution
End of MS life 5 billion years from now Core
energy production - H shell Outer layers will
start to expand
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H.R. Diagram Stellar Evolution
Sun in Red Giant Phase Core becoming degenerate,
hotter Outer layers expanding
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H.R. Diagram Stellar Evolution
Helium Flash Core no longer degenerate Helium
fusion
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H.R. Diagram Stellar Evolution
Steady Helium Fusion C, O core growing
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H.R. Diagram Stellar Evolution
Helium runs out Degeneracy returns Uh-oh! What
next? Larger mass - more fusion Smaller mass -
end of the road
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No 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
lose event - outer layers blown away! Star enters
into its Planetary Nebula Stage
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Planetary Nebula Formation
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Cats Eye Nebula
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H.R. Diagram Evolutionary Track
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White Dwarfs
Small - Radius Earths Radius Hot - Temperature
typically 10,000s K
Dense ? 1,000,000 grams/cm3 ? 1 Ford SUV/cm3
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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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Stars with a masses less than about 8 solar
masses.
Main Sequence Star ? Red Giant ? Planetary Nebula
? White Dwarf
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White Dwarf in a binary system..
White Dwarf
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White Dwarf in a binary system..
Accretion Disk
White Dwarf
Roche Lobes
Evolving Star
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White Dwarf in a binary system..
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Nova - Close binary white dwarf system 100,000
L? Bright for days or weeks Has the potential to
repeat
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