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The Evolution of Low Mass Stars to Helium Ignition
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
The Hertzsprung-Russell Diagram
The Main Sequence
1Mʘ, Z=0.02
Nuclear burning
The Main Sequence - Convection
5Mʘ, Z=0.02
Nuclear burning - convection
Shell burning
Shell burning II
Shell burning III
The Giant Branch
First Dredge-up
First Dredge-up II
First Dredge-up III
First Dredge-up IV
First Dredge-up V
The Luminosity Bump
Problems!
Lithium
Carbon
12C/13C
Nitrogen
Oxygen
Sodium
Gratton et al.
(2000)
-1
0
1
2
Log L/Lsol
3
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...
Helium ignition
Eventually the star becomes hot enough to
ignite He
In stars of >2 solar masses, this happens nondegenerately
For the low-mass stars, the ignition occurs
under degenerate conditions...
...we get a Helium Flash
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...
Helium Flash
At 108 K, He begins to burn
3 12C +
Burning releases energy
The temperature gets hotter
The burning gets faster
More energy is released....
Off centre ignition
Huge Energy Release!
But none of it reaches the surface :-(
On to core He-burning