Helium flash 507854 218816948 2008-06-12T08:41:41Z DOI bot 6652755 Citation maintenance. Initiated by [[User:Julianonions|Julianonions]]. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. A '''helium flash''' is the sudden beginning of [[helium]] [[nuclear fusion|fusion]] in the core of intermediate mass [[star]]s of less than about 2.25 [[solar mass]]es, or on the surface of an [[Accretion (astrophysics)|accreting]] [[white dwarf]] star. They may also occur in the outer layers of larger stars in shell flashes. ==Core helium flash== For a [[star]] with a mass less than 2.25 solar masses, the ''core helium flash'' occurs when the core runs out of [[hydrogen]], and the [[gas pressure]] is no longer sufficient to counter the [[gravitational collapse]]. This causes the star to start contracting. During the contraction the core becomes hotter and hotter until it causes the outer layers to expand outwards initiating the [[red giant]] stage. As the star continues contracting due to gravity, it eventually becomes compressed enough that it becomes [[Degenerate matter|degenerate]] matter. This degeneracy pressure is finally sufficient to stop further collapse of the most central material. As the rest of core continues to contract and the temperature continues to rise, a temperature (~100×10<sup>6</sup> K)is reached at which the helium can start to fuse, and so helium ignition occurs. The explosive nature of the helium flash arises from its taking place in [[degenerate matter]]. When degeneracy pressure (which is purely a function of density) dominates thermal pressure (proportional to the product of density and temperature), the total pressure is only weakly dependent on temperature. Thus, once the temperature reaches 100–200 million [[kelvin]]s and [[helium fusion]] begins using the [[triple-alpha process]], the temperature rapidly increases as degenerate matter is a good conductor of heat, which further increases the helium fusion rate and expands the reaction region, but the pressure does not increase, 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 increased temperature again renders thermal pressure 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. | coauthors = R. K. Wallace | title = The core helium flash and surface abundance anomalies | journal = Astrophysical Journal | date = 1987 |url = http://adsabs.harvard.edu/full/1987ApJ...317..724D | doi = 10.1086/165319 }}</ref> Stars with greater than about 2.25 solar masses start to burn helium before their core becomes degenerate and so do not exhibit this type of helium flash. ===Core helium flash on binary white dwarfs=== When hydrogen gas is accreted onto a ''white dwarf'' from a binary companion star, the hydrogen usually fuses to form helium. This helium can build up to form a shell near the surface of the star. When the mass of helium becomes sufficiently large, a helium flash can occur, with runaway fusion causing a [[nova]]. ==Shell helium flash== '''Shell helium flashes''' are a similar helium ignition event, although not necessarily dependent on 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 continues to burn in a thin shell around this core. The shell of helium is not large enough to raise the material above it, and so cannot expand. Thus there is no expansion related cooling of the burning shell, so the temperature rapidly rises. This leads to a thermal pulse, rapidly releasing the energy built and allowing [[s-process]] reactions to occur. This pulse may last a few hundred years and are thought to occur periodically every 10,000 to 100,000 years. <ref>{{Cite journal | volume = 247 | issue = Part 1 | last = Wood | first = P. R. | coauthors = D. M. Zarro | title = Helium-shell flashing in low-mass stars and period changes in mira variables | journal = Astrophysical Journal | date = 1981 |url = http://adsabs.harvard.edu/full/1981ApJ...247..247W }}</ref> == External links == *[http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v496n1/37173/sc4.html Modelling helium shell flashes on accreting white dwarfs] ==References== <references/> [[Category:Stellar evolution]] [[Category:Astrophysics]] [[de:Helium-Blitz]] [[fr:Flash de l'hélium]] [[he:הבזק הליום]] [[ml:ഹീലിയം ഫ്ലാഷ്]] [[pl:Błysk helowy]] [[ru:Гелиевая вспышка]] [[zh:氦閃]]