An Etymological Dictionary of Astronomy and Astrophysics
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A series of → nuclear reactions taking place in stars in which → carbon, → nitrogen, and → oxygen are used to transform → hydrogen into → helium. In → massive stars the carbon cycle is the dominant process of energy generation, whereas in → low-mass stars such as the Sun, the → proton-proton chain of reactions converts hydrogen into helium. The carbon cycle starts and ends with carbon-12, which acts as a catalyst in the sequential production of helium from hydrogen; neutrinos and gamma rays are also produced.

→ CNO; → cycle.