The CNO cycle is the nuclear fusion process in stars that involves proton fusion of carbon, nitrogen and oxygen as a catalyst. This occurs in stars with temperatures about 15 million Kelvin.
The CNO cycle provides the nuclear reaction that stars use to create light. In the fusion process, the isotopes bond to create the alpha particle, electrons and positrons that interact to produce gamma rays. The stars that undergo carbon fusion do so continuously through a series of four steps or chemical reactions. Stars below 15 million Kelvin undergo hydrogen fusion without a catalyst. The sun undergoes both carbon fusion and hydrogen fusion to create helium.