A helium flash is the sudden beginning of helium fusion in the core of intermediate mass stars, or on the surface of an accreting white dwarf star. Its explosive nature arises from its taking place in degenerate matter. When degeneracy pressure (which is purely a function of density) dominates thermal pressure (proportional to the product of density and temperature), the total pressure is only weakly dependent on temperature. Thus, once the temperature reaches 100–200 MK and helium fusion begins, the temperature rapidly increases, which further increases the helium fusion rate and expands the reaction region, but the pressure does not increase, so there is no stabilizing (and cooling) expansion of the core. This runaway reaction quickly climbs to about 100 billion times the star's normal energy production (for a few seconds) until the increased temperature again renders thermal pressure dominant, eliminating the degeneracy. For the case of an intermediate mass star, the core helium flash occurs when the core runs out of hydrogen, gravitational collapse causes the star to start contracting. During the contraction the core becomes hotter and hotter until it causes the outer layers to expand outwards initiating the red giant stage. As the star continues contracting due to gravity, it eventually becomes degenerate. The degeneracy is lifted as the star's temperatures rises and helium burning starts with near explosive consequences. When hydrogen gas is accreted onto a white dwarf from a binary companion star, the hydrogen usually fuses to form helium. This helium can build up to form a shell near the surface of the star. When the mass of helium becomes sufficiently large, a helium flash can occur, with runaway fusion causing a nova. Shell helium flashes is a similar Helium ignition, although not necessarily dependent on degenerate matter, occurring periodically in Asymptotic Giant Branch stars in a shell outside the core.
