The Helium Flash

We have seen that thermonuclear ignition under degenerate conditions has very different character from thermonuclear ignition in a normal gas. The ignition of helium burning in red giant stars occurs differently for star cores that have reached sufficient density for the electrons to become degenerate than for those cases where the electrons are not degenerate.

Ignition under Nondegenerate Conditions
If the ignition is under non-degenerate conditions, the triple-alpha process turns on rather smoothly and the star core stabilizes because it now has an energy source to counteract gravity. Calculations indicate that for red giants with a mass of 3 solar masses or more the core will not be degenerate at ignition and the core begins to burn helium to carbon by the triple-alpha reaction when the temperature reaches about 150 million K.

Degenerate Ignition: the Helium Flash
On the other hand, for stars of less than about 3 solar masses the core is likely to be degenerate when the conditions are reached to initiate the triple-alpha reaction. Thus, when it initiates it provokes a thermonuclear reaction in degenerate matter. Calculations indicate that the entire core is engulfed in a runaway thermonuclear explosion within a matter of seconds, the temperature of the core shoots up to perhaps 200 million K before it begins to moderate, and the energy release within this sudden event can correspond to 100 billion solar luminosities. This is called the helium flash. Rather remarkably, this enormous outpouring of energy is almost entirely absorbed in the greatly extended outer envelope and there is little outward indication that such a cataclysm has taken place in the core.

Termination of Red Giant Branch Evolution
As the temperature rises in the helium flash, it modifies the gas in the core from a degenerate gas to a more normal gas. Therefore, after a short period the runaway moderates and the triple-alpha reaction proceeds as in the case for heavier stars, with helium burning steadily to carbon at about 150 million K. For both the degenerate and nondegenerate ignitions, the onset of stable triple-alpha burning signals the termination of the red giant branch (RGB) and the beginning of the horizontal branch (HB) of red giant evolution.