CNO CYCLE
The CNO (carbon-nitrogen-oxygen) cycle is one of the fusion responses by which stars convert hydrogen into helium, the other being the proton-proton chain. Although the proton-proton chain is more important in stars of solar mass or less, theoretical models show that the CNO cycle is the dominant energy source in more massive stars. The CNO process was proposed by Carl von Weizsäcker and Hans Bethe independently in 1938 and 1939, respectively.

In the CNO cycle, four protons fuse with isotopes of carbon, nitrogen, and oxygen that act as speculators to produce an alpha particle, two positrons, and two neutrinos. The positrons will always instantaneously annihilate with electrons, releasing energy in the form of gamma radiation. The neutrinos escape the star with some energy. The isotopes of carbon, nitrogen, and oxygen are for all intents and purposes a nucleus that will undergo a number of transformations in an endless cycle, recycling itself.
Although the total number of "catalytic" CNO nuclei remains constant throughout the cycle, the relative proportions of the nuclei change during stellar evolution. When the cycle reaches equilibrium, the ratio of 12C/13C nuclei reaches 3.5, and 14N becomes the most numerous nucleus, regardless of the initial composition. During the evolution of a star, episodes of convective mixing carry material on which the CNO cycle has operated from the interior of the star to the surface, altering the observed composition of the star. It is observed that red giants have lower ratios of 12C/13C and 12C/14N than main sequence stars, something that is considered proof of the generation of nuclear energy in stars by hydrogen fusion.

The presence of elements heavier than carbon, nitrogen and oxygen places an upper limit on the maximum size of massive stars at approximately 150 solar masses. It is thought that the early, "metal-poor" universe could have had stars of up to 250 solar masses without interference from the CNO cycle.