Energy Generation The CNO Cycle

In a low‐mass star like the Sun, energy is generated from the conversion of hydrogen to helium in the proton‐proton cycle. In higher‐mass stars, to balance the greater gravitational pull requires higher pressures, hence higher central temperatures. At higher temperatures, thermonuclear reactions other than those of the proton‐proton cycle become important. For stars of mass greater than about two solar masses, in fact, a different cycle of thermonuclear reactions involving the elements carbon, nitrogen, and oxygen dominates the total energy production. This is the carbon‐nitrogen‐oxygen cycle (or CNO cycle, for short):