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):
A Brief History of Astronomy
The Structure of Stars
