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{{More citations needed|date=March 2016}}
The '''alpha process''', also known as the '''alpha ladder''', is one of two classes of [[nuclear fusion]] reactions by which stars convert [[helium]] into heavier elements, the other being the [[triple-alpha process]].<ref name=narlikar>{{cite book|last=Narlikar|first=Jayant V|title=From Black Clouds to Black Holes|date=1995|publisher=[[World Scientific]]|isbn=978-9810220334|url=https://books.google.com/books?id=0_gmjz-L70EC&pg=PA94|page=94}}</ref> The triple-alpha process consumes only helium, and produces carbon.  After enough carbon has accumulated, the reactions below take place, all consuming only helium and the product of the previous reaction.

:<math chem>\begin{array}{ll}
\ce{_6^{12}C + _2^4He  -> _{8}^{16}O + \gamma }& E=7.16\ \mathrm{MeV}
\\
\ce{_8^{16}O + _2^4He  -> _{10}^{20}Ne + \gamma }& E=4.73\ \mathrm{MeV}
\\
\ce{_{10}^{20}Ne + _2^4He  -> _{12}^{24}Mg + \gamma }& E=9.32\ \mathrm{MeV}
\\
\ce{_{12}^{24}Mg + _2^4He  -> _{14}^{28}Si + \gamma }& E=9.98\ \mathrm{MeV}
\\
\ce{_{14}^{28}Si + _2^4He  -> _{16}^{32}S + \gamma }& E=6.95\ \mathrm{MeV}
\\
\ce{_{16}^{32}S + _2^4He  -> _{18}^{36}Ar + \gamma}& E=6.64\ \mathrm{MeV}
\\
\ce{_{18}^{36}Ar + _2^4He  -> _{20}^{40}Ca + \gamma}& E=7.04\ \mathrm{MeV}
\\
\ce{_{20}^{40}Ca + _2^4He  -> _{22}^{44}Ti + \gamma}& E=5.13\ \mathrm{MeV}
\\
\ce{_{22}^{44}Ti + _2^4He  -> _{24}^{48}Cr + \gamma}& E=7.70\ \mathrm{MeV}
\\
\ce{_{24}^{48}Cr + _2^4He  -> _{26}^{52}Fe + \gamma}& E=7.94\ \mathrm{MeV}
\\
\ce{_{26}^{52}Fe + _2^4He  -> _{28}^{56}Ni + \gamma}& E=8.00\ \mathrm{MeV}
\end{array}</math>
E&nbsp;is the energy produced by the reaction, released primarily as  [[gamma rays]] ({{mvar|&gamma;}}).

It is a common misconception that the above sequence ends at <math>\mathrm{_{28}^{56}Ni}</math> (or <math>\mathrm{_{26}^{56}Fe}</math>, which is a decay product of <math>\mathrm{_{28}^{56}Ni}</math><ref name=":0">{{Cite journal|last=Fewell|first=M. P.|date=1995-07-01|title=The atomic nuclide with the highest mean binding energy|url=http://adsabs.harvard.edu/abs/1995AmJPh..63..653F|journal=American Journal of Physics|volume=63|pages=653–658|doi=10.1119/1.17828|issn=0002-9505}}</ref>) because it is the most stable [[nuclide]] - i.e., it has the highest [[nuclear binding energy]] per [[nucleon]], and production of heavier nuclei requires energy (is [[endothermic]]) instead of releasing it ([[exothermic]]). [[I|<math>\mathrm{_{28}^{62}Ni}</math>]] ([[Nickel-62]]) is actually the most stable nuclide.<ref>{{Cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/nucbin2.html#c1|title=The Most Tightly Bound Nuclei|website=hyperphysics.phy-astr.gsu.edu|access-date=2019-02-21}}</ref> However, the sequence ends at <math>\mathrm{_{28}^{56}Ni}</math> because conditions in the stellar interior cause the competition between [[photodisintegration]] and the alpha process to favor photodisintegration around [[iron]],<ref name=":0" /><ref>{{Cite journal|last=Burbidge|first=E. Margaret|last2=Burbidge|first2=G. R.|last3=Fowler|first3=William A.|last4=Hoyle|first4=F.|date=1957-10-01|title=Synthesis of the Elements in Stars|url=https://link.aps.org/doi/10.1103/RevModPhys.29.547|journal=Reviews of Modern Physics|volume=29|issue=4|pages=547–650|doi=10.1103/RevModPhys.29.547}}</ref> leading to more <math>\mathrm{_{28}^{56}Ni} </math> being produced than <math>\mathrm{_{28}^{62}Ni}</math>.

All these reactions have a very low rate at the temperatures and densities in stars and therefore do not contribute significantly to a star's energy production; with elements heavier than [[neon]] ([[atomic number]] > 10), they occur even less easily due to the increasing [[Coulomb barrier]].

{{anchor|Alpha elements}}'''Alpha process elements''' (or '''alpha elements''') are so-called since their most abundant isotopes are integer multiples of four, the mass of the helium nucleus (the [[alpha particle]]); these isotopes are known as [[alpha nuclide]]s. Stable alpha elements are: [[Carbon|C]], [[Oxygen|O]], [[Neon|Ne]], [[Magnesium|Mg]], [[Silicon|Si]], and [[Sulfur|S]]; [[Argon|Ar]] and [[Calcium|Ca]] are [[observationally stable]]. They are synthesized by alpha capture prior to the [[Silicon burning process|silicon fusing]] process, a precursor to [[Type II supernova]]e. Silicon and calcium are purely alpha process elements. Magnesium can be burned by [[proton capture]] reactions. As for oxygen, some authors{{Which|date=January 2011}} consider it an alpha element, while others do not. Oxygen is surely an alpha element in low-[[metallicity]] [[population II star]]s. It is produced in Type II supernovas and its enhancement is well correlated with an enhancement of other alpha process elements. Sometimes carbon and nitrogen are considered alpha process elements, since they are synthesized in nuclear alpha-capture reactions.

The abundance of alpha elements in stars is usually expressed in a logarithmic manner:
:<math chem> [\alpha/\ce{Fe}] = \log_{10}{\left(\frac{N_{\alpha}}{N_\ce{Fe}}\right)_{Star}} - \log_{10}{\left(\frac{N_{\alpha}}{N_\ce{Fe}}\right)_{Sun}} </math>,
Here <math>N_{\alpha}</math> and <math chem>N_\ce{Fe}</math> are the number of alpha elements and iron nuclei per unit volume. Theoretical [[Galaxy formation and evolution|galactic evolution]] models predict that early in the universe there were more alpha elements relative to iron. Type II supernovae mainly synthesize oxygen and the alpha-elements (Ne, Mg, Si, S, Ar, Ca and Ti) while [[Type Ia supernova]]e mainly produce elements of the [[iron peak]] ([[Titanium|Ti]], [[Vanadium|V]], [[Chromium|Cr]], [[Manganese|Mn]], [[Iron|Fe]], [[Cobalt|Co]] and [[Nickel|Ni]]) but also alpha-elements.

==References==
{{reflist}}

== External links ==
* [https://arxiv.org/abs/0705.4511 The Age, Metallicity and Alpha-Element Abundance of Galactic Globular Clusters from Single Stellar Population Models]

{{Nuclear processes}}
{{Star}}

[[Category:Nuclear fusion]]
[[Category:Nucleosynthesis]]
[[Category:Stellar physics]]