Helium Flash

A helium flash is the runaway fusion of helium in the core of low mass stars of less than about 2.25 solar masses and greater than about 0.5 solar mass, or on the surface of an accreting white dwarf star. A helium flash occurs in these situations because the helium is degenerate, meaning it is supported against gravity by quantum mechanical pressure rather than thermal pressure. Thus an increase in the temperature in the material undergoing fusion does not act to expand the material and by doing so cool it, and there is no regulation of the rate of fusion. It ends when the material is heated to the point where thermal pressure again becomes dominant, and the material then expands and cools. A partially analogous but nonrunaway process also occur in the outer layers of larger stars in shell flashes.