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CNO cycle

From Academic Kids

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. While the proton-proton chain is more important in stars the size of the sun or less, 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:

12C + 1H 13N + γ
13N 13C + e+ + νe
13C + 1H 14N + γ
14N + 1H 15O + γ
15O 15N + e+ + νe
15N + 1H 12C + 4He

The net result of the cycle is to fuse four protons into an alpha particle plus two positrons and two neutrinos, releasing energy in the form of gamma rays. The carbon, oxygen, and nitrogen nuclei serve as catalysts and are regenerated.

In a minor branch of the reaction, occuring 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:

15N + 1H 16O + γ
16O + 1H 17F + γ
17F 17O + e+ + νe
17O + 1H 14N + 4He

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.

See also

de:Bethe-Weizsäcker-Zyklus fr:cycle carbone-azote-oxygène nl:Koolstof-stikstofcyclus pl:Cykl węglowo-azotowo-tlenowy

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