Helium flash

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.
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The ignition of the nuclear fusion of helium in the centre of stars which have left the main sequence . The increase in the output of energy caused by this fusion bloats the star into a red giant