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The CNO cycle (for carbon–nitrogen–oxygen) 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. Unlike the latter, the CNO cycle is a catalytic cycle. Theoretical models suggest 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 starts at temperatures around (4 megakelvins), making it the dominant energy source in smaller stars. A self-maintaining CNO chain starts at approximately , but its energy output rises much more rapidly with increasing temperatures. At approximately , the CNO cycle starts becoming the dominant source of energy.〔 〕 The Sun has a core temperature of around , and only of nuclei 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 + 2 → + 2 + 2 + 2 + 3 + 24.7 MeV → + 2 + 3 + 26.7 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. One nucleus goes to become carbon, nitrogen, and oxygen isotopes through a number of transformations in an endless loop. == Cold CNO cycles == Under typical conditions found in stars, catalytic hydrogen burning by the CNO cycles is limited by proton captures. Specifically, the timescale for beta decay of the radioactive nuclei produced is faster than the timescale for fusion. Because of the long timescales involved, the cold CNO cycles convert hydrogen to helium slowly, allowing them to power stars in quiescent equilibrium for many years. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「CNO cycle」の詳細全文を読む スポンサード リンク
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