The combination or fusion of three alpha particles (helium nuclei 4He) to form a carbon nucleus (12C) is known as the triple alpha process.
- The fusion process is not at all simple since there does not exist a stable configuration with an atomic mass of 8 (8Be) that is formed by the fusion of two 4He nuclei. The lifetime of 8Be is a very short 3 × 10-16 seconds.
- However if this unstable 8Be nucleus is struck by another 4He nuclei it is possible to form 12C (and a gamma ray). This can occur because the lifetime of 8Be is actually longer than the mean collision or scattering time of helium nuclei at temperatures around 108K which are needed to make this fusion process proceed. In 1952 Edwin Salpeter proposed this process as the way to form 12C.
- Not long afterwards Fred Hoyle suggested that the fusion of 8Be and 4He would be greatly enhanced if the 12C nucleus possessed an energy level near to the combined energies of the two nuclei involved. The subsequent reaction would then be a resonant reaction. This resonant energy level was found by experiments at the Kellogg Radiation Laboratory at CalTech.
The triple alpha process will occur in red giant stars that have left the main sequence (and have consumed their core hydrogen) and have core temperatures of 108K and higher. Once 12C has been formed it is possible with temperatures around 6 × 108 K to continue forming heavier nuclei by the combination of two 12C nuclei to make 16O , 20Ne, 24Mg and with temperatures around 109K the combination of two 16O nuclei can make 28Si, 31P, 31S and 32S.