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.
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.