Helium flash
From Free net encyclopedia
A Helium flash is the sudden beginning of helium burning in the core of intermediate mass stars, or on the surface of an accreting white dwarf star.
For the case of an intermediate mass star, the 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.