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The CNO (carbon-nitrogen-oxygen) cycle is one of two fusion reactions by which stars convert hydrogen to helium, the other being the proton-proton chain. The proton-proton chain is more important in stars the mass of the sun or less. Only 1.7% of 4He nuclei being produced in the Sun are born in the CNO cycle. However theoretical models show that the CNO cycle is the dominant source of energy in heavier stars. The CNO process was proposed in 1938 by Hans Bethe. The reactions of the CNO cycle are The net result of the cycle is to fuse four protons into an alpha particle plus two positrons (annihilating with electrons and releasing energy in the form of gamma rays) plus two neutrinos which are escaping from the star with some part of energy. The carbon, oxygen, and nitrogen nuclei serve as catalysts and are regenerated. In a minor branch of the reaction, occurring in the Sun core just 0.04% of the time, the final reaction shown above does not produce 12C and 4He, but instead produces 16O and a photon and continues as follows: Like the carbon, nitrogen, and oxygen involved in the main branch, the fluorine produced in the minor branch is merely catalytic and at steady state, does not accumulate in the star. The main branch of the CNO cycle is known as CNO-I, the minor branch as CNO-II. There exist also two subdominant branches of CNO-III and CNO-IV which are significant only for heavy stars. They are started when the last reaction in CNO-II results in oxygen-18 and gamma instead of nitrogen-14 and alpha: 17O + 1H → 18O + γ. While the total number of "catalytic" CNO nuclei is conserved in the cycle, in stellar evolution the relative proportions of the nuclei are altered. When the cycle is run to equilibrium, the ratio of the 12C/13C nuclei is driven to 3.5, and 14N becomes the most numerous nucleus, regardless of initial composition. During a star's evolution, convective mixing episodes bring material in which the CNO cycle has operated from the star's interior to the surface, altering the observed composition of the star. Red giant stars are observed to have lower 12C/13C and 12C/14N ratios than main sequence stars, which is considered to be proof of nuclear energy generation in stars by hydrogen fusion.
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