The helium flash is the onset of runaway helium burning in the core of a low-mass star (such as the Sun).

 

The helium flash happens in the hydrogen-exhausted core of a star that has become a red giant. When gravitational pressure has raised the temperature of the dormant helium core to a temperature of about 100 million K, the helium nuclei start to undergo thermonuclear reactions. Once the helium burning has started, the temperature climbs rapidly (without a cooling, stabilizing expansion), and the extreme sensitivity of the nuclear reaction rate to temperature causes the helium-burning process to accelerate. This in turn raises the temperature, which further accelerates the helium burning, until a point is reached at which the thermal pressure expands the core and thereby limits the flash.

 

The helium flash can only occur when the helium core is less than the 1.4-solar-mass Chandrasekhar limit and thus it is restricted to fairly low-mass stars.

 

Helium shell flash

The helium shell flash is one of a series of helium burning episodes in the thin helium shell that surrounds the dormant carbon core of an asymptotic giant branch star; the helium burning shell does not generate energy at a constant rate but instead produces energy primarily in short flashes. During a flash, the region just outside the helium-burning shell becomes unstable to convection; the resultant mixing probably leads to the s-process as well as to the upward movement of carbon produced by helium burning. The overheating from a flash also causes an expansion of the star's upper layers, followed by a collapse, leading to large-scale pulsations.

 

Final helium flash object

A final helium flash object is an evolved star that, in descending the white dwarf cooling track, undergoes a final thermal pulse when its helium shell ignites causing it to expand rapidly to high luminosity and become a born-again planetary nebula. This phase of stellar evolution is extremely short, lasting only a few decades to a few centuries and would rarely be observed. A handful of final helium flash object candidates have been identified, including Sakurai's Object, FG Sagittae, and V605 Aquilae.