Definition
The CNO cycle is a series of nuclear reactions converting H to He and using C, N, and O isotopes as catalysts; thus, the total amount of these latter species is not affected by the operation. There are three such cycles, each converting four protons to one 4He nucleus and releasing about 6.6 MeV/nucleon or 5 1018 erg/g. At temperatures higher than 17 106 K, encountered in stars more massive than 1.3–1.5 M⊙, depending on metallicity, rotation, etc., the CNO cycle is the dominant source of energy production, while in lower-mass stars, the proton-proton (p-p) chains dominate. Although the sum of C + N + O abundances is conserved, 12C and 16O are converted to 14N, thus 14N is the main product of the CNO cycle from the chemical evolution point of view.
History
The CNO cycle is also called the Bethe-Weizsäcker cycle. Hans Bethe won the 1967 Nobel Prize in physics for his 1938 discovery of energy production in stars.
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© 2014 Springer-Verlag Berlin Heidelberg
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Prantzos, N. (2014). CNO Cycle. In: Amils, R., et al. Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-27833-4_307-5
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DOI: https://doi.org/10.1007/978-3-642-27833-4_307-5
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Publisher Name: Springer, Berlin, Heidelberg
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