Helium fusion
The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon.
Helium accumulates in the core of stars as a result of the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle. Further nuclear fusion reactions of helium with hydrogen or another alpha particle produce lithium-5 and beryllium-8 respectively. Both products are highly unstable and decay, almost instantly, back into smaller nuclei, unless a third alpha particle fuses with a beryllium before that time to produce a stable carbon-12 nucleus.
When a star runs out of hydrogen to fuse in its core, it begins to collapse until the central temperature rises to 108 K, six times hotter than the sun's core. At this temperature and density, alpha particles are able to fuse rapidly enough (the half-life of 5Li is 3.7×10−22 s and that of 8Be is 6.7×10−17 s) to produce significant amounts of carbon and restore thermodynamic equilibrium in the core
The net energy release of the process is 7.273 MeV (1.166 pJ).
As a side effect of the process, some carbon nuclei fuse with additional helium to produce a stable isotope of oxygen and energy:
See alpha process for more details about this reaction and further steps in the chain of stellar nucleosynthesis.
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