View source for Helium flash

{{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]]s, 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 |last=Taylor |first=David |date=June 2012 |title=The End Of The Sun |url=http://faculty.wcas.northwestern.edu/~infocom/The%20Website/end.html |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>

==Subflashes==
Subflashes are pulsational instabilities that occur after the main helium flash. They are driven by stars that do not have good convective or radiative boundaries.<ref name="Battich2018">{{Cite journal |last1=Battich |first1=Tiara |last2=Bertolami |first2=Marcelo M. Miller |last3=Córsico |first3=Alejandro H. |last4=Althaus |first4=Leandro G. |date=June 2018 |title=Pulsational instabilities driven by the ∈ mechanism in hot pre-horizontal branch stars - I. The hot-flasher scenario |url=https://www.aanda.org/articles/aa/full_html/2018/06/aa31463-17/aa31463-17.html |journal=[[Astronomy & Astrophysics]] |volume=614 |pages=A136 |arxiv=1801.07287 |bibcode=2018A&A...614A.136B |doi=10.1051/0004-6361/201731463 |issn=0004-6361}}</ref> Subflashes can last several hours to days and can occur for many years with each subsequent flash generally being weaker.<ref name="Battich2018" /> Subflashes can be detected by applying [[fourier transform]]s to the light curve data.<ref>{{Cite journal |last=Balona |first=Luis A. |date=2020-10-29 |title=The Hot Limit of Solar-like Oscillations From Kepler Photometry |journal=[[Frontiers in Astronomy and Space Sciences]] |volume=7 |page=85 |article-number=529025 |doi=10.3389/fspas.2020.529025 |doi-access=free |arxiv=2008.06299 |bibcode=2020FrASS...7...85B |issn=2296-987X}}</ref>

==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. Hydrogen fusion continues in a shell surrounding the core, which causes continued accumulation of helium in the core. This makes the core denser, but its 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 gravitational pressure and achieve 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 {{val|p=≈|1|e=8|ul=K}}, at which point 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 100–200 million [[kelvin]]s 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 [[positive feedback loop|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 |last1=Deupree |first1=R. G. |last2=Wallace |first2=R. K. |date=1987 |title=The core helium flash and surface abundance anomalies |journal=[[Astrophysical Journal]] |volume=317 |pages=724–732 |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 before its core becomes 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. |date=November 1996 |title=A Reexamination of the Core Helium Flash |journal=[[The Astrophysical Journal]] |volume=471 |issue=1 |pages=377–384 |bibcode=1996ApJ...471..377D |citeseerx=10.1.1.31.44 |doi=10.1086/177976 |s2cid=15585754}}</ref><ref>{{Cite thesis |last1=Mocák |first1=M. |title=Multidimensional hydrodynamic simulations of the core helium flash in low-mass stars |date=February 2009 |type=PhD. Thesis |publisher=Technische Universität München |bibcode=2009PhDT.........2M}}</ref>

In a one solar mass star, the helium flash is estimated to release about {{val|5|e=41|ul=J}},<ref name="Edwards1969">{{cite journal |last=Edwards |first=A. C. |date=1969 |title=The Hydrodynamics of the Helium Flash |journal=Monthly Notices of the Royal Astronomical Society |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 |last1=Khokhlov |first1=A. |last2=Müller |first2=E. |last3=Höflich |first3=P. |date=1993 |title=Light curves of Type IA supernova models with different explosion mechanisms |journal=[[Astronomy and Astrophysics]] |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.

==Binary white dwarfs==

When hydrogen gas is accreted onto a ''white dwarf'' from a binary companion star, the hydrogen can fuse to form helium for a narrow range of accretion rates, but most systems develop a layer of hydrogen over the degenerate white dwarf interior. This hydrogen can build up to form a shell near the surface of the star. When the mass of hydrogen becomes sufficiently large, runaway fusion causes a [[nova]]. In a few binary systems where the hydrogen fuses on the surface, the mass of helium built up can burn in an unstable helium flash. In certain binary systems the companion star may have lost most of its hydrogen and donate helium-rich material to the compact star. Note that [[X-ray burster|similar flashes]] occur on neutron stars.{{citation needed|date=February 2016}}

==Helium shell flash<span class="anchor" id="Shell helium flash"></span><span class="anchor" id="shell helium flash"></span>==
'''Helium shell flashes''' are a somewhat analogous but much less violent, nonrunaway helium ignition event, taking place in the absence of degenerate matter. They occur periodically in [[asymptotic giant branch]] stars in a shell outside the core. This is late in the life of a star in its giant phase. The star has burnt most of the helium available in the core, which is now composed of carbon and oxygen. Helium fusion continues in a thin shell around this core, but then turns off as helium becomes depleted. This allows hydrogen fusion to start in a layer above the helium layer. After enough additional helium accumulates, helium fusion is reignited, leading to a thermal pulse which eventually causes the star to expand and brighten temporarily (the pulse in luminosity is delayed because it takes a number of years for the energy from restarted helium fusion to reach the surface<ref name = "Wood"/>). Such pulses may last a few hundred years, and are thought to occur periodically every 10,000 to 100,000 years.<ref name = "Wood">{{Cite journal
| volume = 247
| issue = Part 1
| last1 = Wood
| first1 = P. R.
| last2 = Zarro
| first2 = D. M.
| title = Helium-shell flashing in low-mass stars and period changes in mira variables
| journal = [[Astrophysical Journal]]
| date = 1981
| bibcode = 1981ApJ...247..247W
| doi = 10.1086/159032
| pages = 247 
}}</ref> 
After the flash, helium fusion continues at an exponentially decaying rate for about 40% of the cycle as the helium shell is consumed.<ref name = "Wood"/> Thermal pulses may cause a star to shed circumstellar shells of gas and dust.{{citation needed|date=February 2016}}

==See also==
* {{annotated link|Carbon detonation}}

==References==
<references/>

{{white dwarf}}
{{Star}}

{{DEFAULTSORT:Helium Flash}}
[[Category:Concepts in astrophysics]]
[[Category:Exotic matter]]
[[Category:Helium]]
[[Category:Nucleosynthesis]]
[[Category:Stellar evolution]]