Carbon-nitrogen-oxygen cycles (astrophysics)
Caughlan, Georgeanne R. Department of Physics, Montana State University, Bozeman, Montana.
Last reviewed:November 2019
- Carbon-nitrogen cycle
- CNO bicycle
- Other CNO cycles
- Related Primary Literature
- Additional Reading
A group of nuclear reactions that involve the interaction of protons (nuclei of hydrogen atoms, designated by 1H) with carbon, nitrogen, and oxygen nuclei. The cycle involving only isotopes of carbon and nitrogen is well known as the carbon-nitrogen (CN) cycle. These cycles are thought to be the main source of energy in main-sequence stars with mass 40% or more in excess of that of the Sun. Completion of any one of the cycles results in consumption of four protons (4 1H) and the production of a helium (4He) nucleus plus two positrons (e+) and two neutrinos (ν). The two positrons are annihilated with two electrons (e−), and the total energy release is 26.73 MeV. Approximately 1.7 MeV is released as neutrino energy and is not available as thermal energy in the star. The energy E = 26.73 MeV arises from the mass difference between four hydrogen atoms and the helium atom, and is calculated from the Einstein mass-energy equation E = Δmc2, where Δm is the mass difference and c2 is the square of the velocity of light. Completion of a chain can be thought of as conversion of four hydrogen atoms into a helium atom. Because the nuclear fuel that is consumed in these processes is hydrogen, they are referred to as hydrogen-burning processes by means of the carbon-nitrogen-oxygen (CNO) cycles. See also: Solar neutrinos
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