CNO cycle

In <a href="/facts/Astrophysics/krFPRWLA">astrophysics</a>, the carbon–nitrogen–oxygen (CNO) cycle, sometimes called Bethe–Weizsäcker cycle, after <a href="/facts/Hans_Albrecht_Bethe/X6bkxRRI">Hans Albrecht Bethe</a> and <a href="/facts/Carl_Friedrich_von_Weizs%C3%A4cker/HJFv48U0">Carl Friedrich von Weizsäcker</a>, is one of the two known sets of <a href="/facts/Nuclear_fusion/hP0gLDqj">fusion</a> <a href="/facts/Nuclear_reaction/202yClUr">reactions</a> by which <a href="/facts/Star/PFbd6ICx">stars</a> convert <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a> to <a href="/facts/Helium/yRul93TG">helium</a>, the other being the <a href="/facts/Proton%E2%80%93proton_chain_reaction/UUDAR9yW">proton–proton chain reaction</a> (p–p cycle), which is more efficient at the <a href="/facts/Sun/CadPCVHh">Sun</a>'s core temperature. The CNO cycle is hypothesized to be dominant in stars that are more than 1.3 times as <a href="/facts/Solar_mass/j6Auhku8">massive as the Sun</a>.
Unlike the proton-proton reaction, which consumes all its constituents, the CNO cycle is a <a href="/facts/Catalytic_cycle/At7x3sNe">catalytic cycle</a>. In the CNO cycle, four <a href="/facts/Proton/clCjepXP">protons</a> fuse, using <a href="/facts/Carbon/ukrgQexo">carbon</a>, <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a>, and <a href="/facts/Oxygen/acyn6o8r">oxygen</a> isotopes as catalysts, each of which is consumed at one step of the CNO cycle, but re-generated in a later step. The end product is one <a href="/facts/Alpha_particle/dzNMs9WP">alpha particle</a> (a <a href="/facts/Stable_nuclide/vC0VVvqN">stable</a> <a href="/facts/Helium/yRul93TG">helium</a> nucleus), two <a href="/facts/Positron/HgV4zBzA">positrons</a>, and two <a href="/facts/Electron_neutrino/e5hQPWIQ">electron neutrinos</a>.
There are various alternative paths and catalysts involved in the CNO cycles, but all these cycles have the same net result:

4 11H   +   2 e−
  →   42He   +   2 e+   +   2 e−   +   2 νe   +   3 γ   +   24.7 <a href="/facts/Electronvolt/YMT1fqX9">MeV</a>
  →   42He   +   2 νe   +   7 γ   +   26.7 MeV
The positrons will almost instantly <a href="/facts/Electron%E2%80%93positron_annihilation/ca6HdHfu">annihilate with electrons</a>, releasing energy in the form of <a href="/facts/Gamma_ray/5Drf913J">gamma rays</a>. The neutrinos escape from the star carrying away some energy. One nucleus goes on to become carbon, nitrogen, and oxygen isotopes through a number of transformations in a repeating cycle.

The proton–proton chain is more prominent in stars the mass of the Sun or less. This difference stems from temperature dependency differences between the two reactions; pp-chain reaction starts at temperatures around 4×106 <a href="/facts/Kelvin/E3trzC4C">K</a> (4 megakelvin), making it the dominant energy source in smaller stars. A self-maintaining CNO chain starts at approximately 15×106 <a href="/facts/Kelvin/E3trzC4C">K</a>, but its energy output rises much more rapidly with increasing temperatures so that it becomes the dominant source of energy at approximately 17×106 <a href="/facts/Kelvin/E3trzC4C">K</a>.
The Sun has a <a href="/facts/Solar_core/V4wKBVzp">core</a> temperature of around 15.7×106 <a href="/facts/Kelvin/E3trzC4C">K</a>, and only 1.7% of <a href="/facts/Helium-4/vNsL5qhC">4He</a> nuclei produced in the Sun are
born in the CNO cycle.
The CNO-I process was independently proposed by <a href="/facts/Carl_Friedrich_von_Weizs%C3%A4cker/HJFv48U0">Carl von Weizsäcker</a> and <a href="/facts/Hans_Bethe/X6bkxRRI">Hans Bethe</a> in the late 1930s.
The first reports of the experimental detection of the neutrinos produced by the CNO cycle in the Sun were published in 2020 by the <a href="/facts/Borexino/JkQbEVJP">BOREXINO</a> collaboration. This was also the first experimental confirmation that the Sun had a CNO cycle, that the proposed magnitude of the cycle was accurate, and that von Weizsäcker and Bethe were correct.

CNO cycle open-in-new

CNO cycle