CNO cycle

noun

\ ˌsē-ˌen-ˈō-

Definition of CNO cycle

: the process by which large stars convert elemental hydrogen into helium through a series of nuclear fusion reactions To become a supernova, the progenitor star had to be at least eight times our Sun's mass, so it must have fused hydrogen into helium via the CNO cycle, a reaction that uses carbon, nitrogen, and oxygen as catalysts.— Wayne J. Parker, Sky & Telescope, January 1995

Note: Stars that are roughly twice the size of the Sun or larger convert hydrogen by means of the CNO cycle, in which hydrogen nuclei combine with and then separate from carbon, nitrogen and oxygen in a specific repeating loop that generates both helium and extra energy. Smaller stars also fuse hydrogen into helium, but the process is different and does not involve other elements.