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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 ...
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.
It dominates in stars with masses less than or equal to that of the Sun, [2] whereas the CNO cycle, the other known reaction, is suggested by theoretical models to dominate in stars with masses greater than about 1.3 solar masses. [3]
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 massive stars (above 10 M ☉) [38] the rate of energy generation by the CNO cycle is very sensitive to temperature, so the fusion is highly concentrated at the core. Consequently, there is a high temperature gradient in the core region, which results in a convection zone for more efficient energy transport. [ 31 ]
In massive stars (greater than about 1.5 M ☉), the core temperature is above about 1.8×10 7 K, so hydrogen-to-helium fusion occurs primarily via the CNO cycle. In the CNO cycle, the energy generation rate scales as the temperature to the 15th power, whereas the rate scales as the temperature to the 4th power in the proton-proton chains. [2]
For stars at 1.5 M ☉ where the core temperature reaches 18 MK, half the energy production comes from the CNO cycle and half from the pp chain. [5] The CNO process is more temperature-sensitive than the pp chain, with most of the energy production occurring near the very center of the star.
The counter-intuitive existence of lithium-rich red giant stars that have gone through first dredge-up may be explained by scenarios such as mass transfer. [1] The second dredge-up The second dredge-up occurs in stars with 4–8 solar masses. When helium fusion comes to an end at the core, convection mixes the products of the CNO cycle. [2]