File:CNO Cycle.svg

The CNO cycle (for carbon-nitrogen-oxygen), or sometimes Bethe-Weizsäcker-cycle, is one of two sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton-proton chain. Theoretical models show that the CNO cycle is the dominant source of energy in stars more massive than than about 1.5 times the mass of the sun. The proton-proton chain is more important in stars the mass of the sun or less. This difference stems from temperature dependency differences between the two reactions; pp-chain reactions start occurring at temperatures around 4×106 Template:Val/unitswithlink, making it the dominant force in smaller stars. The CNO chain starts occurring at approximately 13×106 Template:Convert/ScientificValue/LoffAonSoffT[citation needed], but its energy output rises much faster with increasing temperatures. At approximately 17×106 Template:Convert/ScientificValue/LoffAonSoffT, the CNO cycle starts becoming the dominant source of energy. This occurs in stars with masses at least 1.3 times the solar mass.[1] The Sun has a core temperature of around 15.7×106 Template:Convert/ScientificValue/LoffAonSoffT and only 1.7% of 4He nuclei being produced in the Sun are born in the CNO cycle. The CNO process was proposed by Carl von Weizsäcker[2] and Hans Bethe[3] independently in 1938 and 1939, respectively.

In the CNO cycle, four protons fuse, using carbon, nitrogen and oxygen isotopes as a catalyst, to produce one alpha particle, two positrons and two electron neutrinos. The positrons will almost instantly annihilate with electrons, releasing energy in the form of gamma rays. The neutrinos escape from the star carrying away some energy. The carbon, nitrogen, and oxygen isotopes are in effect one nucleus that goes through a number of transformations in an endless loop.

CNO-I Edit

The main reactions of the CNO cycle are 126C137N136C147N158O157N126C:[4]

126C 11H → 137N γ   1.95 MeV
137N   → 136C e+ νe 2.22 MeV
136C 11H → 147N γ   7.54 MeV
147N 11H → 158O γ   7.35 MeV
158O   → 157N e+ νe 2.75 MeV
157N 11H → 126C 42He   4.96 MeV

where the Carbon-12 nucleus used in the first reaction is regenerated in the last reaction.


In a minor branch of the reaction, occurring in the Sun's core just 0.04% of the time, the final reaction shown above does not produce carbon-12 and an alpha particle, but instead produces oxygen-16 and a photon and continues 157N168O179F178O147N158O157N:

157N 11H → 168O γ   12.13 MeV
168O 11H → 179F γ   0.60 MeV
179F   → 178O e+ νe 2.76 MeV
178O 11H → 147N 42He   1.19 MeV
147N 11H → 158O γ   7.35 MeV
158O   → 157N e+ νe 2.75 MeV

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.

OF Cycle Edit

This subdominant branch is significant only for massive stars. The reactions are started when one of the reactions in CNO-II results in fluorine-18 and gamma instead of nitrogen-14 and alpha, and continues 178O189F188O199F168O179F178O:

178O 11H → 189F γ   5.61 MeV
189F   → 188O e+ νe 1.656 MeV
188O 11H → 199F γ   7.994 MeV
199F 11H → 168O 42He   8.114 MeV
168O 11H → 179F γ   0.60 MeV
179F   → 178O e+ νe 2.76 MeV

Note that all these cycles have the same net result:

4 11H  →  42He  +  2 e+  +  2 νe  +  γ  +  26.8 MeV

Use in astronomy Edit

While the total number of "catalytic" CNO nuclei is conserved in the cycle, in stellar evolution the relative proportions of the nuclei are altered. When the cycle is run to equilibrium, the ratio of the carbon-12/carbon-13 nuclei is driven to 3.5, and nitrogen-14 becomes the most numerous nucleus, regardless of initial composition. During a star's evolution, convective mixing episodes bring material in which the CNO cycle has operated from the star's interior to the surface, altering the observed composition of the star. Red giant stars are observed to have lower carbon-12/carbon-13 and carbon-12/nitrogen-14 ratios than main sequence stars, which is considered to be convincing evidence for the operation of the CNO cycle.

The presence of the heavier elements carbon, nitrogen and oxygen places an upward bound on the maximum size of massive stars to approximately 150 solar masses. It is thought that the "metal-poor" early universe could have had stars, called Population III stars, up to 250 solar masses without interference from the CNO cycle.

See alsoEdit

External linksEdit


  1. Schuler, Simon C.; King, Jeremy R.; The, Lih-Sin (August 2009). "Stellar Nucleosynthesis in the Hyades Open Cluster". The Astrophysical Journal 701 (1): 837–849. doi:10.1088/0004-637X/701/1/837. 
  2. von Weizsäcker, C. F. (1938), "Über Elementumwandlungen in Innern der Sterne II (Element Transformation Inside Stars, II)", Physikalische Zeitschrift 39: 633–46 
  3. Bethe, H. A. (1939). "Energy Production in Stars". Physical Review 55 (5): 434–56. doi:10.1103/PhysRev.55.434, 
  4. Kenneth S. Krane (1988), Introductory Nuclear Physics, John Wiley & Sons, p. 537, ISBN 0-471-80553-X 
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