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.