Alpha process

The alpha process, also known as alpha capture or the alpha ladder, is one of two classes of nuclear fusion reactions by which stars convert helium into heavier elements. The other class is a cycle of reactions called the triple-alpha process, which consumes only helium, and produces carbon.^[1] The alpha process most commonly occurs in massive stars and during supernovae.

Both processes are preceded by hydrogen fusion, which produces the helium that fuels both the triple-alpha process and the alpha ladder processes. After the triple-alpha process has produced enough carbon, the alpha-ladder begins and fusion reactions of increasingly heavy elements take place, in the order listed below. Each step only consumes the product of the previous reaction and helium. The later-stage reactions which are able to begin in any particular star, do so while the prior stage reactions are still under way in outer layers of the star.

${\displaystyle {\begin{array}{ll}{\ce {~{}_{6}^{12}C\ ~~+{}_{2}^{4}He\ ->~{}_{8}^{16}O\ \ ~+\gamma ~,}}&E={\mathsf {7.16\ MeV}}\\{\ce {~{}_{8}^{16}O\ ~~+{}_{2}^{4}He\ ->{}_{10}^{20}Ne\ \ +\gamma ~,}}&E={\mathsf {4.73\ MeV}}\\{\ce {{}_{10}^{20}Ne\ ~+{}_{2}^{4}He\ ->{}_{12}^{24}Mg\ +\gamma ~,}}&E={\mathsf {9.32\ MeV}}\\{\ce {{}_{12}^{24}Mg\ +{}_{2}^{4}He\ ->{}_{14}^{28}Si\ ~~+\gamma ~,}}&E={\mathsf {9.98\ MeV}}\\{\ce {{}_{14}^{28}Si\ ~~+{}_{2}^{4}He\ ->{}_{16}^{32}S\ \ ~~~+\gamma ~,}}&E={\mathsf {6.95\ MeV}}\\{\ce {{}_{16}^{32}S\ ~~~+{}_{2}^{4}He\ ->{}_{18}^{36}Ar\ ~\ +\gamma ~,}}&E={\mathsf {6.64\ MeV}}\\{\ce {{}_{18}^{36}Ar\ ~+{}_{2}^{4}He\ ->{}_{20}^{40}Ca\ \ +\gamma ~,}}&E={\mathsf {7.04\ MeV}}\\{\ce {{}_{20}^{40}Ca\ +{}_{2}^{4}He\ ->{}_{22}^{44}Ti\ ~~+\gamma ~,}}&E={\mathsf {5.13\ MeV}}\\{\ce {{}_{22}^{44}Ti\ ~+{}_{2}^{4}He\ ->{}_{24}^{48}Cr\ ~+\gamma ~,}}&E={\mathsf {7.70\ MeV}}\\{\ce {{}_{24}^{48}Cr\ +{}_{2}^{4}He\ ->{}_{26}^{52}Fe\ ~\ +\gamma ~,}}&E={\mathsf {7.94\ MeV}}\\{\ce {{}_{26}^{52}Fe\ +{}_{2}^{4}He\ ->{}_{28}^{56}Ni\ ~\ +\gamma ~,}}&E={\mathsf {8.00\ MeV}}\end{array}}}$

The energy produced by each reaction, E, is mainly in the form of gamma rays (γ), with a small amount taken by the byproduct element, as added momentum.

It is a common misconception that the above sequence ends at ${\displaystyle \,{}_{28}^{56}\mathrm {Ni} \,}$ (or ${\displaystyle \,{}_{26}^{56}\mathrm {Fe} \,}$, which is a decay product of ${\displaystyle \,{}_{28}^{56}\mathrm {Ni} \,}$^[2]) because it is the most tightly bound nuclide – i.e., the nuclide with the highest nuclear binding energy per nucleon – and production of heavier nuclei would consume energy (be endothermic) instead of release it (exothermic). ${\displaystyle \,{}_{28}^{62}\mathrm {Ni} \,}$ (Nickel-62) is actually the most tightly bound nuclide in terms of binding energy^[3] (though ${\displaystyle {}^{56}{\textrm {Fe}}}$ has a lower energy or mass per nucleon). The reaction ${\displaystyle {}^{56}{\textrm {Fe}}+{}^{4}{\textrm {He}}\rightarrow {}^{60}{\textrm {Ni}}}$ is actually exothermic, and indeed adding alphas continues to be exothermic all the way to ${\displaystyle \ {}_{50}^{100}\mathrm {Sn} \ }$,^[4] but nonetheless the sequence does effectively end at iron. The sequence stops before producing elements heavier than nickel because conditions in stellar interiors cause the competition between photodisintegration and the alpha process to favor photodisintegration around iron.^[2][5] This leads to more ${\displaystyle \,{}_{28}^{56}\mathrm {Ni} \,}$ being produced than ${\displaystyle \,{}_{28}^{62}\mathrm {Ni} ~.}$

All these reactions have a very low rate at the temperatures and densities in stars and therefore do not contribute significant energy to a star's total output. They occur even less easily with elements heavier than neon (Z > 10) due to the increasing Coulomb barrier.