The CNO cycle (for carbon–nitrogen–oxygen) is one of two sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain. Unlike the proton–proton chain reaction, the CNO cycle is a catalytic cycle. Theoretical models show that the CNO cycle is the dominant source of energy in stars more massive than about 1.3 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 K, making it the dominant energy source in smaller stars. A self-maintaining CNO chain starts occurring at approximately 15×106 K, but its energy output rises much more rapidly with increasing temperatures. At approximately 17×106 K, the CNO cycle starts becoming the dominant source of energy. The Sun has a core temperature of around 15.7×106 K and only 1.7% of 4He nuclei being produced in the Sun are born in the CNO cycle. The CNO-I process was independently proposed by Carl von Weizsäcker and Hans Bethe 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. Although there are various paths and catalysts involved in the CNO cycles, simply speaking all these cycles have the same net result:
- 4 1
1H → 4
2He + 2 e+ + 2 ν
e + 3 γ + 26.8 MeV
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.
Other articles related to "cno cycle, cno, cycle":
... fusion occurs primarily via the CNO cycle ... In the CNO cycle, the energy generation rate scales as the temperature to the 17th power, whereas the rate scales as the temperature to the 4th power in the proton-proton chains ... Due to the strong temperature sensitivity of the CNO cycle, the temperature gradient in the inner portion of the star is steep enough to make the core convective ...
... While the total number of "catalytic" CNO nuclei are 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 ... which is considered to be convincing evidence for the operation of the CNO cycle ...
... The pp-chain reaction cycle is relatively insensitive to temperature, so this hydrogen burning process can occur in up to a third of the star's radius and occupy half ... In each complete fusion cycle, the p-p chain reaction releases about 26.2 MeV ... In higher mass stars, the dominant process is the CNO cycle, which is a catalytic cycle that uses nuclei of carbon, nitrogen and oxygen as intermediaries to produce a helium nucleus ...
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