triple alpha process

The triple alpha process (helium burning) is a fusion reaction in stars turning helium into carbon. This happens in older stars with abundant helium when the core collapses enough to produce sufficient temperature and density. Helium fuses into beryllium-8 which then fuses with another helium atom to produce carbon:

4    4     8
 He + He -> Be (-93.7 keV)
2    2     4

8    4     12
 Be + He ->  C (+7.367 MeV)
4    2      6

At about 100 million K, helium fuses faster than beryllium-8 decays and this process becomes significant. When this occurs, there is carbon to fuse with helium into oxygen (which can be thought of as the first step in the alpha process), or at a higher temperature, carbon burning may take place. It is not well established how readily carbon and helium fuse, which is termed the C12(α,γ)O16 reaction, α for the helium nucleus (i.e., alpha particle) and γ for a resulting gamma ray. The more readily this occurs, the more oxygen is produced, which is thought to influence subsequent developments significantly, e.g., in stellar evolution.