Animation of the triple alpha process, a stellar nuclear reaction that burns helium to create carbon and oxygen in the core of a star. The triple alpha process occurs late in a star's life, when most of its core hydrogen has been converted to helium. The helium-helium fusion reaction is unlikely, and its product, beryllium-8, decays almost immediately back to two helium nuclei. However, as the star's core starts to collapse due to lack of hydrogen fuel to support it, beryllium-8 is produced faster than it can decay. Helium-helium fusion releases energy as a gamma ray (yellow wave), and the immediate fusion of a further helium nucleus releases another gamma ray, and forms the more stable carbon-12. This can also fuse with another helium-4, forming the similarly stable oxygen-16. This process forms the vast majority of the carbon and oxygen in the universe.