(Submitted April 09, 1999)
What exactly causes helium flashes in stars at the point when they
begin to burn helium rather than hydrogen?
A helium flash occurs because
the core of the star is in what is known as a "degenerate" state. This
means that the core has contracted so much that the pressure of the
electron shells of the atoms making up the core prevent the core from
contracting further. Under normal gas conditions (i.e. NOT a degenerate
state), an increase in the temperature of the core would cause an increase
in core pressure resulting in the core expanding and the temperature then
dropping. This state is known as hydrostatic equilibrium. With a
degenerate core, the temperature increases but the pressure doesn't.
This extra energy ignites the helium creating run-away nuclear reactions.
This is what is referred to as a "helium flash." For more details, check
Hope this helps.
Allie Hajian &
for Ask an Astrophysicist