[[Image:CNO_Cycle.png|300px|thumb|CNO cycle]]The [[Sun]], and stars as massive as the Sun or less massive, commonly use a [[nuclear fusion]] process called the '''proton-proton chain reaction''' to produce [[energy]]. A full description of that process appears [[Sun#Energy production and transport|here]].
In 1938 and 1989, two physicists, Carl F. von Weizsäcker<ref name=Weiz>Von Weizsäcker, Carl F. ''Physik. Zeitsch.'' 39:633, 1938.</ref> and Hans Bethe<ref name=Bethe>Bethe, Hans A. "[http://prola.aps.org/abstract/PR/v55/i5/p434_1 Energy Production in Stars]." ''Physics Review'' 55(5):434-456, 1939. {{doi|10.1103/PhysRev.55.434}} Accessed June 27, 2008.</ref> independently proposed a [[nuclear fusion]] process, the '''Carbon-Nitrogen-Oxygen cycle''', by which stars more massive than the [[sun]] produce energy. In this process, stars convert [[hydrogen]] to [[helium]] using [[carbon]], [[nitrogen]], and [[oxygen]] as catalysts. The reaction also produces two [[positron]]s and two [[neutrino|electron neutrino]]s.<ref name=Krane>Krane, Kenneth S. ''Introductory Nuclear Physics''. New York: John Wiley and Sons, 1988, p. 537. ISBN 9780471805533</ref>
The equations for the cycle are as follows: