Editing Helium flash (section)

==Red giants==
[[File:White Dwarf Resurrection.jpg|thumb|[[Sakurai's Object]] is a [[white dwarf]] undergoing a helium shell flash.<ref>{{cite web|title=White Dwarf Resurrection|url=http://www.eso.org/public/images/potw1531a/|access-date=3 August 2015}}</ref>]]
During the [[red giant]] phase of [[stellar evolution]] in stars with less than 2.0 {{Solar mass}}, the [[nuclear fusion]] of hydrogen ceases in the core as it is depleted, leaving a helium-rich core. While fusion of hydrogen continues in the star's shell causing a continuation of the accumulation of helium in the core, making the core denser, the temperature is still unable to reach the level required for helium fusion, as happens in more massive stars. Thus the thermal pressure from fusion is no longer sufficient to counter the gravitational collapse and create the [[hydrostatic equilibrium]] found in most stars. This causes the star to start contracting and increasing in temperature until it eventually becomes compressed enough for the helium core to become [[degenerate matter]]. This degeneracy pressure is finally sufficient to stop further collapse of the most central material but the rest of the core continues to contract and the temperature continues to rise until it reaches a point ({{val|p=≈|1|e=8|ul=K}}) at which the helium can ignite and start to fuse.<ref name=Hansen2004>{{cite book|title=Stellar Interiors - Physical Principles, Structure, and Evolution|url=https://archive.org/details/stellarinteriors00hans_446|url-access=limited|last1=Hansen|first1=Carl J.|last2=Kawaler|first2=Steven D.|last3=Trimble|first3=Virginia|isbn=978-0387200897 |date=2004|edition=2|publisher=Springer|pages=[https://archive.org/details/stellarinteriors00hans_446/page/n73 62]–5}}</ref><ref name=Seeds2012>{{cite book|title=Foundations of Astronomy|last1=Seeds|first1=Michael A.|last2=Backman|first2=Dana E.|pages=249–51|date=2012|edition=12|publisher=[[Cengage Learning]]|isbn=978-1133103769}}</ref><ref name=Karttunen2007>{{cite book|title=Fundamental Astronomy|url=https://archive.org/details/fundamentalastro00kart_346|url-access=limited|isbn=978-3540341437|edition=5|page=[https://archive.org/details/fundamentalastro00kart_346/page/n251 249]|editor-first=Hannu|editor-last=Karttunen|editor2-first=Pekka|editor2-last=Kröger|editor3-first=Heikki|editor3-last=Oja|editor4-first=Markku|editor4-last=Poutanen|editor5-first=Karl Johan|editor5-last=Donner|publisher=Springer|date=2007-06-27}}</ref>

The explosive nature of the helium flash arises from its taking place in degenerate matter. Once the temperature reaches 100 million–200 million [[kelvin]] and helium fusion begins using the [[triple-alpha process]], the temperature rapidly increases, further raising the helium fusion rate and, because degenerate matter is a good [[Thermal conduction|conductor of heat]], widening the reaction region.

However, since degeneracy pressure (which is purely a function of density) is dominating thermal pressure (proportional to the product of density and temperature), the total pressure is only weakly dependent on temperature.  Thus, the dramatic increase in temperature only causes a slight increase in pressure, so there is no stabilizing 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 temperature increases to the point that thermal pressure again becomes dominant, eliminating the degeneracy. The core can then expand and cool down and a stable burning of helium will continue.<ref>{{Cite journal| volume = 317| pages = 724–732| last = Deupree| first = R. G.|author2=R. K. Wallace| title = The core helium flash and surface abundance anomalies| journal = Astrophysical Journal| date = 1987|bibcode = 1987ApJ...317..724D| doi = 10.1086/165319| doi-access = free}}</ref>

A star with mass greater than about 2.25 {{Solar mass}} starts to burn helium without its core becoming degenerate, and so does not exhibit this type of helium flash. In a very low-mass star (less than about 0.5 {{Solar mass}}), the core is never hot enough to ignite helium. The degenerate helium core will keep on contracting, and finally becomes a [[White dwarf#Stars with very low mass|helium white dwarf]].

The helium flash is not directly observable on the surface by electromagnetic radiation. The flash occurs in the core deep inside the star, and the net effect will be that all released energy is absorbed by the entire core, causing the degenerate state to become nondegenerate. Earlier computations indicated that a nondisruptive mass loss would be possible in some cases,<ref name="Deupree1984">{{cite journal|last1= Deupree|first1= R. G.|title= Two- and three-dimensional numerical simulations of the core helium flash|journal= The Astrophysical Journal|volume= 282|year= 1984|pages= 274|doi=10.1086/162200|bibcode= 1984ApJ...282..274D|doi-access= free}}</ref> but later star modeling taking neutrino energy loss into account indicates no such mass loss.<ref name="Deupree1996">{{cite journal|last1= Deupree|first1=R. G.|title= A Reexamination of the Core Helium Flash|journal= The Astrophysical Journal|volume=471|issue= 1|date= 1996-11-01|pages= 377–384|doi= 10.1086/177976|bibcode= 1996ApJ...471..377D|citeseerx= 10.1.1.31.44|s2cid=15585754 }}</ref><ref>{{Cite thesis |bibcode = 2009PhDT.........2M|title = Multidimensional hydrodynamic simulations of the core helium flash in low-mass stars|last1 = Mocák|first1 = M|year = 2009 |type=PhD. Thesis |publisher=Technische Universität München}}</ref>

In a one solar mass star, the helium flash is estimated to release about {{val|5|e=41|ul=J}},<ref name="Edwards19690">{{cite journal | author=Edwards, A. C.|title= The Hydrodynamics of the Helium Flash | journal= Monthly Notices of the Royal Astronomical Society | date=1969 | volume=146 |issue= 4 | pages=445–472 | bibcode= 1969MNRAS.146..445E|doi = 10.1093/mnras/146.4.445 | doi-access= free }}</ref> or about 0.3% of the energy release of a {{val|1.5|e=44|ul=J}} [[type Ia supernova]],<ref name="Khokhlov1993">{{cite journal | author1=Khokhlov, A. |author2=Müller, E. |author3=Höflich, P. | title= Light curves of Type IA supernova models with different explosion mechanisms | journal= Astronomy and Astrophysics | date=1993 | volume=270 | issue=1–2 | pages=223–248 | bibcode= 1993A&A...270..223K}}</ref> which is triggered by an analogous [[Carbon detonation|ignition of carbon fusion]] in a carbon–oxygen white dwarf.