Editing Helium flash (section)

{{short description|Brief thermal runaway nuclear fusion in the core of low-mass stars}}
[[File:Helium flash.svg|300px|thumb|right|Fusion of helium in the core of low-mass stars.]]
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 [[star]]s (between 0.8 [[solar mass]]es ({{Solar mass|link=yes}}) and 2.0 {{Solar mass}}<ref>{{cite book|type=lecture notes|title=Stellar Structure and Evolution|first=Onno|last=Pols|date=September 2009|chapter-url=https://astro.uni-bonn.de/~nlanger/siu_web/ssescript/new/chapter9.pdf|archive-url=https://web.archive.org/web/20190520071013/https://astro.uni-bonn.de/~nlanger/siu_web/ssescript/new/chapter9.pdf|archive-date=20 May 2019|chapter=Chapter 9: Post-main sequence evolution through helium burning}}</ref>) 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 [[Accretion (astrophysics)|accreting]] [[white dwarf]] stars.

Low-mass stars do not produce enough [[gravity|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 mechanics|quantum mechanical]] pressure rather than [[ideal gas law|thermal pressure]].  Subsequent hydrogen shell fusion further increases the mass of the core until it reaches temperature of approximately 100 million [[kelvin]], which is hot enough to initiate helium fusion (or "helium burning") in the core.

However, a property of degenerate matter is that increases in temperature do not produce an increase in the pressure of the matter until the thermal pressure becomes so very high that it exceeds degeneracy pressure. In main sequence stars, [[hydrostatic equilibrium|thermal expansion]] regulates the core temperature, but in degenerate cores, this does not occur. Helium fusion increases the temperature, which increases the fusion rate, which further increases the temperature in a runaway reaction which quickly spans the entire core. This produces a flash of very intense helium fusion that lasts only a few minutes,{{r|End}} but during that time, produces energy at a rate comparable to the entire [[Milky Way]] galaxy.{{r|End}}

In the case of normal low-mass stars, the vast energy release causes much of the core to come out of degeneracy, allowing it to thermally expand.  This consumes most of the total energy released by the helium flash,{{r|End}} and any left-over energy is absorbed into the star's upper layers. Thus the helium flash is mostly undetectable by observation, and is described solely by astrophysical models. After the core's expansion and cooling, the star's surface rapidly cools and contracts in as little as 10,000 years until it is roughly 2% of its former radius and luminosity. 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.<ref name=End>{{cite web |url=http://faculty.wcas.northwestern.edu/~infocom/The%20Website/end.html |title=The End Of The Sun |first=David |last=Taylor |website=[[Northwestern University]] |quote=almost all the energy of the flash is absorbed by the titanic weight-lifting necessary to lift the core out of its white-dwarf condition.}}</ref>