In stars with central temperatures greater than 15 million Kelvin, carbon fusion is thought to take over the dominant role rather than hydrogen fusion. The main theme of the carbon cycle is the adding of protons, but after a carbon-12 nucleus fuses with a proton to form nitrogen-13, one of the protons decays with the emission of a positron and a neutrino to form carbon-13. Two more proton captures produce nitrogen-14 and then oxygen-15. Another neutron decay leaves nitrogen-15. Another proton capture produces oxygen-16 which emits an energetic alpha particle to return to carbon-12 to repeat the cycle. This last reaction is the main source of energy in the cycle for the fueling of the star.

While this process is not a significant part of the sun's fuel cycle, a star like Sirius with somewhat more than twice the mass of the sun derives almost all of its power from the carbon cycle. The carbon cycle yields 26.72 MeV per helium nucleus.

Hans Bethe and the Carbon Cycle

For his role in working out the energy source for stars more massive than the sun, the carbon cycle, Hans Bethe received the Nobel Prize in 1967. Bethe was one of the outstanding young scientists who fled Nazi Germany in the 1930's. One of the fascinating stories about Hans Bethe is that after submitting his article about the carbon cycle to the Physical Review, he became aware of a $500 prize for the best unpublished paper about energy production in the stars. He asked Physical Review to return his paper, proceeded to win the prize and paid a finder's fee of $50 to Robert Marshak who had told him about it. Bethe recounts "I used part of the prize to help my mother emigrate. The Nazis were quite willing to let her out, but they wanted $250, in dollars, to release her furniture. Part of the prize money went to liberate my mother's furniture."