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 CNO cycle may also be the dominant cause of nitrogen and oxygen production.

In actuality there is not only CNO cycle but three possible cycles which are astrophysically important. The main CNO cycle looks like this:

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

The cycle results in the fusion of four hydrogen nuclei (1H, protons) into a single helium nucleus (4He, alpha particle), which supplies energy to the star in accordance with Einstein's equation. Ordinary carbon serves as a catalyst in this set of reactions and is regenerated.

See also: Triple-alpha process, Proton-proton chain