triple alpha process

A nuclear fusion reaction in which three helium nuclei (alpha particles) fuse to form a carbon nucleus, thereby releasing energy. Two helium nuclei fuse to form an unstable isotope of beryllium, which under conditions of sufficient temperature and pressure fuses with a third helium nucleus to form carbon before it decays. Triple alpha processes take place in stars in which large quantities of helium have accumulated as the product of proton-proton chain and carbon cycle reactions.