CNO cycle

A CNO cycle is a fusion reaction within stars that turns hydrogen into helium, combining the hydrogen with carbon, nitrogen or oxygen (CNO) in chains of reactions that produce helium, along with the same type of carbon, nitrogen, or oxygen atom (any of the three effectively a catalyst). The CNO cycle is one of two reaction-types that turn hydrogen into helium, the other being the proton-proton chain (PP chain), which requires no such catalyst. The more massive and hotter the star, the more CNO-cycle fusion it generates, and for stars of more than 1.3 solar masses, it is the primary reaction. 15 million K is enough to trigger it, and at 17 million K, it becomes more prevalent than the PP chain.

There are 7 CNO cycles, CNO-I, CNO-II, CNO-III, and CNO-IV, the others termed hot cycles, HCNO-I, HCNO-II, and HCNO-III. Example (6 steps of CNO-I):

12    1     13
  C +  H →    N + γ       + 1.9  MeV
 6    1      7

13          13     +
  N      →    C + e  + ν  + 1.20 MeV (9.965 minute half-life)
 7           6          e

13    1     14
  C +  H →    N + γ       + 7.54 MeV
 6    1      7

14    1     15
  N +  H →    O + γ       + 7.35 MeV
 7    1      8

15          15     +
  O      →    N + e + ν   + 1.73 MeV (122.24 second half-life)
 8           7         e

15    1     12    4
  N +  H →    O +  H      + 4.96 MeV
 7    1      6    2

• γ - gamma-ray photon.
• ν_e - electron neutrino.
• e⁺ - positron.
• number MeV - net kinetic energy added to the resulting particles (contributing to temperature).

This cycle can commence with any of the above steps, the most likely being those using the common stable isotopes, ¹²C or ¹⁴N. Some of the other cycles similarly incorporate ¹⁶O. The CNO cycle was proposed by Carl von Weizsacker and Hans Bethe in 1938 and 1939.