What is "helium flash"

n. (context astronomy English) The sudden beginning of the burning of helium in the core of stars of intermediate mass or on the surface of an accrete white dwarf.

A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 solar masses and 2.0 ) during their red giant phase (the Sun is predicted to experience a flash 1.2 billion years after it leaves the main sequence). A much rarer runaway helium fusion process can also occur on the surface of accreting white dwarf stars.

Low mass stars do not produce enough gravitational pressure to initiate normal helium fusion. As the hydrogen in the core is exhausted, some of the helium left behind is instead compacted into degenerate matter, supported against gravitational collapse by quantum mechanical pressure rather than thermal pressure. This increases the density and temperature of the core until it reaches approximately 100 million kelvin, which is hot enough to cause helium fusion (or "helium burning") in the core.

However, a fundamental quality of degenerate matter is that changes in temperature do not change the volume of the matter (until the thermal pressure becomes so incredibly high that it exceeds degeneracy pressure). Unable to regulate the rate of fusion through thermal expansion (as main sequence stars do when they begin to burn too much hydrogen), the helium fusion increases the temperature, which increases the fusion rate, which further increases the temperature in a runaway reaction. This flash of very intense helium fusion lasts only a few minutes, but briefly emits energy at a rate comparable to the entire Milky Way galaxy.

In the case of normal low mass stars, the vast energy release is absorbed by the star's upper layers, and thus is mostly undetectable to observation, described solely by astrophysical models. The process ends when the core material is heated to the point where thermal pressure again becomes dominant, and the material then expands and cools. It is estimated that the electron-degenerate helium core weighs about 40% of the star mass and that 6% of the core is converted into carbon.