Editing Triple-alpha process (section)

==In stars==
[[File:Nuclear energy generation.svg|right|upright=1.25|thumb|Comparison of the energy output (ε) of [[Proton–proton chain reaction|proton–proton]] (PP), [[CNO cycle|CNO]] and '''Triple-α''' fusion processes at different temperatures (T). The dashed line shows the combined energy generation of the PP and CNO processes within a star.]]
[[Helium]] accumulates in the [[stellar core|core]]s of stars as a result of the [[proton–proton chain reaction]] and the [[CNO cycle|carbon–nitrogen–oxygen cycle]].

Nuclear fusion reaction of two helium-4 nuclei produces [[beryllium-8]], which is highly unstable, and decays back into smaller nuclei with a half-life of {{val|8.19e-17|u=s}}, unless within that time a third alpha particle fuses with the beryllium-8 nucleus<ref name=":02">{{Cite book |last1=Bohan |first1=Elise |url=https://www.worldcat.org/oclc/940282526 |title=Big History |last2=Dinwiddie |first2=Robert |last3=Challoner |first3=Jack |last4=Stuart |first4=Colin |last5=Harvey |first5=Derek |last6=Wragg-Sykes |first6=Rebecca |last7=Chrisp |first7=Peter |last8=Hubbard |first8=Ben |last9=Parker |first9=Phillip |collaboration=Writers |date=February 2016 |publisher=[[DK (publisher)|DK]] |others=Foreword by [[David Christian (historian)|David Christian]] |isbn=978-1-4654-5443-0 |edition=1st American |location=[[New York City|New York]] |pages=58 |oclc=940282526 |author-link6=Rebecca Wragg Sykes |author-link7=Peter Chrisp}}</ref> to produce an excited [[Resonance (particle physics)|resonance]] state of [[carbon-12]],{{NUBASE2016|ref}} called the [[Carbon-12#Hoyle state|Hoyle state]], which nearly always decays back into three alpha particles, but once in about 2421.3 times releases energy and changes into the stable base form of carbon-12.<ref>[https://physics.aps.org/articles/v4/38 ''The carbon challenge''], Morten Hjorth-Jensen, Department of Physics and Center of Mathematics for Applications, [[University of Oslo]], N-0316 Oslo, Norway: 9 May 2011, [[Physics (magazine)|''Physics'']] 4, 38</ref> When a star runs out of [[hydrogen]] to fuse in its core, it begins to contract and heat up. If the central temperature rises to 10<sup>8</sup> K,<ref>{{cite book|last1=Wilson|first1=Robert|title=Astronomy through the ages the story of the human attempt to understand the universe|date=1997|publisher=[[Taylor & Francis]]|location=Basingstoke|isbn=9780203212738|chapter=Chapter 11: The Stars – their Birth, Life, and Death}}</ref> six times hotter than the Sun's core, alpha particles can fuse fast enough to get past the beryllium-8 barrier and produce significant amounts of stable carbon-12.

:{|
| {{nuclide|link=yes|Helium|4}} + {{nuclide|Helium|4}} → {{nuclide|link=yes|Beryllium|8}}
| &nbsp;(−0.0918 MeV)
|-
| {{nuclide|Beryllium|8}} + {{nuclide|Helium|4}} → {{nuclide|link=yes|Carbon|12}} + 2{{Subatomic particle|photon|link=yes}}
| &nbsp;(+7.367 MeV)
|}

The net energy release of the process is 7.275 MeV.

As a side effect of the process, some carbon nuclei fuse with additional helium to produce a stable isotope of oxygen and energy:

: {{nuclide|link=yes|Carbon|12}} + {{nuclide|link=yes|Helium|4}} → {{nuclide|link=yes|Oxygen|16}} + {{Subatomic particle|photon|link=yes}} (+7.162 MeV)

Nuclear fusion reactions of helium with hydrogen produces [[lithium-5]], which also is highly unstable, and decays back into smaller nuclei with a half-life of {{val|3.7e-22|u=s}}.

Fusing with additional helium nuclei can create heavier elements in a chain of [[stellar nucleosynthesis]] known as the [[alpha process]], but these reactions are only significant at higher temperatures and pressures than in cores undergoing the triple-alpha process.  This creates a situation in which stellar nucleosynthesis produces large amounts of carbon and oxygen, but only a small fraction of those elements are converted into [[neon]] and heavier elements. Oxygen and carbon are the main "ash" of helium-4 burning.