- Back to Stellar Evolution
When we last saw the Sun it was evolving up the red
giant branch (also known as First Ascent Giant Branch) with
an ever-larger He core that is contracting and heating the
surrounding H-fusion shell.
- So where is this heading?
Eventually the core stops contracting for a Strange Reason .
You might expect the core to heat up to K where He fusion
would start and produce the thermal pressure required to
balance gravity and then work in the thermostat mode. But, this is wrong
- The Red Giant core is supported by Electron Degeneracy.
This comes about because you can only squeeze electrons so close
together and no further because of the Pauli Exclusion
- The Red Giant core is made up of mostly ionized He so
it is a gas of nuclei and .
All subatomic particles have various properties characterized
by ``quantum numbers''. One is ``spin''. The quantum world gets
split into two flavors of particles according to their spin:
- ``spin 1/2'' particles are called Fermions ( are Fermions).
- ``spin 1'' particles are called Bosons (neutrons and He nuclei are Bosons)
- Fermions all obey a law called the Pauli Exclusion Principle:
Here, the x,y,z refer to position and etc refer to momentum.
- At a given temperature (this means give the mean speed of
the ), as the density goes up (packing those closer and closer
together), the gas starts to run out phase space
in which to put . The inability to increase the density
adds up to a form of supporting pressure called Electron Degeneracy Pressure > .
- Electron Degeneracy Pressure is what balances gravity in the
RGB star core and EDP has a strange property in that it
is more-or-less independent of temperature.
- Remember the usual Gas Law:
This provides a saftey value:
- The Usual Gas Law as a Safety Value:
- Suppose the He fusion started in the core. This would release
some heat, the temperature and pressure would go up, the core
expands and cools off and the fusion rate decreases a little bit.
- Alternatively suppose the fusion rates go down. Then gravity
starts to compress the core, the Temperature goes up, fusion
rates go up and the core re-expands.
- But, because EDP is independent of temperature, when
He fusion starts, it is like a bomb going off.
- Core Temperature reaches K.
- A special form of He fusion starts where 3He Carbon
and there is an enormous release of energy and increase in the core temperature.
- Because the pressure in the core is due to degeneracy and is
T independent, the core does NOT expand and cool, but rather there is
a temperature runaway - this is the .
- Although it sounds exciting, the Helium Flash does not
add up to all that much. It runs for a few hours until the
temperature reaches and at this temperature there is
lots of extra phase space available to the electrons and the
degeneracy is ``lifted''. The result is a small hiccup and
the internal structure now looks like a version of a main-sequence
star with . The new equilibrium puts
the star in a spot on the H-R Diagram called the ``horizontal