Astrophysics (Index)About

CNO cycle

(fusion reaction chain turning hydrogen and nitrogen into helium and nitrogen)

A CNO cycle is a fusion reaction within stars that turns hydrogen into helium if nitrogen is present along with sufficient temperature. CNO stands for carbon, nitrogen, oxygen, elements involved in the reaction. It is one of two types of reaction turning hydrogen into helium, the other being the proton-proton chain, which is independent of nitrogen. 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.

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

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

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

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

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

15          15     +
  O      →    N + e + v   + 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

The CNO cycle was proposed by Carl von Weizsacker and Hans Bethe in 1938 and 1939.


(fusion,nuclear,reaction,nucleosynthesis)
Further reading:
http://en.wikipedia.org/wiki/CNO_cycle

Referenced by pages:
carbon (C)
hydrogen burning
main sequence star (MS)
nova (N)
nuclear energy generation rate (ε)
nucleosynthesis
oxygen (O)
proton-proton chain
stellar core
stellar structure

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