The 'CNO cycle' refers to the Carbon-Nitrogen-Oxygen cycle, a process of stellar nucleosynthesis in which stars on the Main Sequence fuse hydrogen into helium via a six-stage sequence of reactions. This sequence proceeds as follows:
Thus, the carbon-12 nucleus used in the initial reaction is regenerated in the final one and hence acts as a catalyst for the whole cycle. The cycle commences once the stellar core temperature reaches 14 × 106 K and is the primary source of energy in stars of mass M > 1.5 M⊙. Stars of lower mass convert hydrogen to helium via an alternative process known as the 'proton-proton chain'.