The Evolution of Low Mass Stars to Helium Ignition

1
The Evolution of Low Mass Stars to Helium Ignition
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Overview

• Look at how structure of a star changes
• How do its abundances change?
• We will look at particular phases in the
  evolution

• Main sequence
• The giant branch
• Helium ignition

3
The Hertzsprung-Russell Diagram
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The Main Sequence
1M?, Z0.02
5
Nuclear burning
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The Main Sequence - Convection
5M?, Z0.02
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Nuclear burning - convection
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Shell burning
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Shell burning II
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Shell burning III
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The Giant Branch
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First Dredge-up
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First Dredge-up II
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First Dredge-up III
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First Dredge-up IV
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First Dredge-up V
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The Luminosity Bump
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Problems!
Lithium
Carbon
12C/13C
Nitrogen
Oxygen
Sodium
Gratton et al. (2000)?
-1
0
1
2
3
Log L/Lsol
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Extra Mixing!

• Material that has undergone nuclear burning
  reaches the surface
• This is not convection!!!
• Some additional mixing process is at work
• We need to circulate material between the burning
  shell and the base of the convective envelope...

20
Helium ignition

• Eventually the star becomes hot enough to ignite
  He
• In stars of gt2 solar masses, this happens
  non-degenerately
• For the low-mass stars, the ignition occurs under
  degenerate conditions...
• ...we get a Helium Flash

21
Degenerate Cores

• As core mass grows, so does the density
• Core becomes electron degenerate e's can't be
  forced into the same states
• Equation of state P P(?)?
• There is no longer any temperature dependence
• No thermostatic control...

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Helium Flash

• At 108 K, He begins to burn
• Burning releases energy
• The temperature gets hotter
• The burning gets faster
• More energy is released....

3? ? 12C ?
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Off centre ignition
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Huge Energy Release!
But none of it reaches the surface -(
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On to core He-burning