Helium flash

A much rarer runaway helium fusion process can also occur on the surface of accreting white dwarf stars.

Low-mass stars do not produce enough gravitational pressure to initiate normal helium fusion.

[2] In the case of normal low-mass stars, the vast energy release causes much of the core to come out of degeneracy, allowing it to thermally expand.

Subflashes can last several hours to days and can occur for many years with each subsequent flash generally being weaker [2].

While fusion of hydrogen continues in the star's shell causing a continuation of the accumulation of helium in the core, making the core denser, the temperature is still unable to reach the level required for helium fusion, as happens in more massive stars.

Thus the thermal pressure from fusion is no longer sufficient to counter the gravitational collapse and create the hydrostatic equilibrium found in most stars.

This causes the star to start contracting and increasing in temperature until it eventually becomes compressed enough for the helium core to become degenerate matter.

[4][5][6] The explosive nature of the helium flash arises from its taking place in degenerate matter.

Once the temperature reaches 100 million–200 million kelvin and helium fusion begins using the triple-alpha process, the temperature rapidly increases, further raising the helium fusion rate and, because degenerate matter is a good conductor of heat, widening the reaction region.

This runaway reaction quickly climbs to about 100 billion times the star's normal energy production (for a few seconds) until the temperature increases to the point that thermal pressure again becomes dominant, eliminating the degeneracy.