The CNO (carbon-nitrogen-oxygen) cycle is one of two fusion reactions by which stars convert hydrogen to helium, the other being the proton-proton chain. While the proton-proton chain is more important in stars the size of the sun or less, theoretical models show that the CNO cycle is the dominant source of energy in heavier stars.

The reactions of the CNO cycle are:

12C + 1H	→	13N + γ
13N	→	13C + e+ + νe
13C + 1H	→	14N + γ
14N + 1H	→	15O + γ
15O	→	15N + e+ + νe
15N + 1H	→	12C + 4He

The net result of the cycle is to fuse four protons into an alpha particle plus two electrons and two neutrinos, releasing energy in the form of gamma rays. The carbon, oxygen, and nitrogen nuclei serve as catalysts and are regenerated.

The reactions in the CNO cycle producing nitrogen and oxygen may be the dominant source of these elements in stars.

See also

• Triple-alpha process
• Proton-proton chain