The explosive nature of the helium flash 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 million kelvins and helium fusion begins using the triple-alpha process, the temperature rapidly increases as degenerate matter is a good conductor of heat, which further increases the helium fusion rate and expands the reaction region, but the pressure does not increase, so there is no stabilizing 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. The core can then expand and cool down and a stable burning of helium will continue.
Stars with greater than about 2.25 solar masses start to burn helium before their core becomes degenerate and so do not exhibit this type of helium flash.
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 are a similar helium ignition event, although not necessarily dependent on degenerate matter. They occur periodically in Asymptotic Giant Branch stars in a shell outside the core. This is late in the life of a star in its giant phase. The star has burnt most of the helium available in the core, which is now composed of carbon and oxygen. Helium continues to burn in a thin shell around this core. The shell of helium is not large enough to raise the material above it, and so cannot expand. Thus there is no expansion related cooling of the burning shell, so the temperature rapidly rises. This leads to a thermal pulse, rapidly releasing the energy built and allowing s-process reactions to occur. This pulse may last a few hundred years and are thought to occur periodically every 10,000 to 100,000 years. Thermal pulses may cause a star to shed circumstellar shells of gas and dust.