Triple-alpha Process

Production of 12C by fusion of 3 4He nuclei (α particles). Helium produced in hydrogen burning cannot undergo fusion reactions because of a stability bottleneck. The two most likely reactions for 4He fusion are:

However, both products are unstable, and decay before they can undergo any further reactions. Only at extremely high temperatures (~108 K), can this stability bottleneck be circumvented by a highly improbable reaction. At these temperatures, 8Be (t½ = 0.067 seconds) is formed at a high enough rate that there is always a very small equilibrium concentration at any one instant. This small concentration of 8Be fuses with 4He yielding an excited state of 12C (indicated as 12C*) that is also unstable. However, a few 12C* emit a γ-ray
quickly enough to become stable before disintegrating. This extremely improbable sequence is called the "triple-alpha process" because the net reaction is:

The triple-alpha process does not occur in main sequence stars like the Sun because their central temperatures are too low. However, in the red giant phase, after many main sequence stars have consumed their H fuel, central temperatures rise high enough to initiate the triple-alpha process. (Further helium capture reactions then convert some C into O.) Thus, much of the energy for red giants comes from fusion of He into C. Helium burning stars occupy a region of the Hertzsprung-Russell diagram known as the horizontal branch.


Image source: http://physics.uoregon.edu/~jimbrau/astr122/Notes/Chapter21.html.