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The CNO cycle (for carbon–nitrogen–oxygen; sometimes called Bethe–Weizsäcker cycle after Hans Albrecht Bethe and Carl Friedrich von Weizsäcker) is one of the two known sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain reaction (p–p cycle), which is more efficient at the Sun's ...
The PP process and the CNO process are equal at around 20 MK. [1] Scheme of the proton–proton branch I reaction The proton–proton chain , also commonly referred to as the p–p chain , is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium .
Comparison of the energy output (ε) of proton–proton (PP), CNO and Triple-α fusion processes at different temperatures (T). The dashed line shows the combined energy generation of the PP and CNO processes within a star. Helium accumulates in the cores of stars as a result of the proton–proton chain reaction and the carbon–nitrogen ...
In higher-mass stars, the dominant energy production process is the CNO cycle, which is a catalytic cycle that uses nuclei of carbon, nitrogen and oxygen as intermediaries and in the end produces a helium nucleus as with the proton–proton chain. [22] During a complete CNO cycle, 25.0 MeV of energy is released.
The other class is a cycle of reactions called the triple-alpha process, which consumes only helium, and produces carbon. [1] The alpha process most commonly occurs in massive stars and during supernovae. Both processes are preceded by hydrogen fusion, which produces the helium that fuels both the triple-alpha process and the alpha ladder ...
The majority of these occur within stars, and the chain of those nuclear fusion processes are known as hydrogen burning (via the proton–proton chain or the CNO cycle), helium burning, carbon burning, neon burning, oxygen burning and silicon burning. These processes are able to create elements up to and including iron and nickel.
Above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of carbon, nitrogen, and oxygen as intermediaries in the CNO cycle that produces helium from hydrogen atoms. Main-sequence stars with more than two solar masses undergo convection in their core regions, which acts to stir up the newly created helium and ...
CNO cycle. The second reaction sequence, in which 4 H nuclei may eventually result in one He nucleus, is called the CNO cycle and generates less than 10% of the total solar energy. This involves carbon atoms which are not consumed in the overall process. The details of this CNO cycle are as follows: