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CNO cycle
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<h2 id="cold-cno-cycles">Cold CNO cycles</h2> <p>Under typical conditions found in stars, catalytic hydrogen burning by the CNO cycles is limited by <a href="/facts/Proton_capture/eLQDChDd">proton captures</a>. Specifically, the timescale for <a href="/facts/Beta_decay/vxTBlckp">beta decay</a> of the <a href="/facts/Radionuclide/5WpCfbdq">radioactive nuclei</a> 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. </p> <h3>CNO-I</h3> <p>The first proposed catalytic cycle for the conversion of hydrogen into helium was initially called the carbon–nitrogen cycle (CN-cycle), also referred to as the Bethe–Weizsäcker cycle in honor of the independent work of <a href="/facts/Carl_Friedrich_von_Weizs%C3%A4cker/HJFv48U0">Carl Friedrich von Weizsäcker</a> in 1937–38<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a><a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> and <a href="/facts/Hans_Bethe/X6bkxRRI">Hans Bethe</a>. Bethe's 1939 papers on the CN-cycle<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a><a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> drew on three earlier papers written in collaboration with <a href="/facts/Robert_Bacher/vKamClXs">Robert Bacher</a> and <a href="/facts/Milton_Stanley_Livingston/SLXfr1jo">Milton Stanley Livingston</a><a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a><a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> and which came to be known informally as <i>Bethe's Bible</i>. It was considered the standard work on nuclear physics for many years and was a significant factor in his being awarded the <a href="/facts/List_of_Nobel_laureates_in_Physics/1gsf7AN5">1967 Nobel Prize in Physics</a>.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> Bethe's original calculations suggested the CN-cycle was the Sun's primary source of energy.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a><a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> This conclusion arose from a belief that is now known to be mistaken, that the <a href="/facts/Abundance_of_the_chemical_elements/CE9WehBl">abundance of nitrogen in the sun</a> is approximately 10%; it is actually less than half a percent.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> The CN-cycle, named as it contains no stable isotope of oxygen, involves the following cycle of transformations:<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> </p> <a href="/facts/Carbon-12/nJYECzEr">126C</a> → <a href="/facts/Nitrogen-13/AMvEc8m7">137N</a> → <a href="/facts/Carbon-13/Iho4LUtW">136C</a> → <a href="/facts/Nitrogen-14/JdTSL0KH">147N</a> → <a href="/facts/Oxygen-15/ektDXb8B">158O</a> → <a href="/facts/Nitrogen-15/JdTSL0KH">157N</a> → 126C <p>This cycle is now understood as being the first part of a larger process, the CNO-cycle, and the main reactions in this part of the cycle (CNO-I) are:<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> </p> <table><tbody><tr><td>126C </td><td>+ </td><td>11H </td><td>→ </td><td>137N </td><td>+ </td><td><a href="/facts/Gamma_ray/5Drf913J">γ</a> </td><td> </td><td> </td><td>+ </td><td>1.95 <a href="/facts/Electronvolt/YMT1fqX9">MeV</a></td></tr><tr><td>137N </td><td> </td><td> </td><td>→ </td><td>136C </td><td>+ </td><td><a href="/facts/Positron/HgV4zBzA">e+</a> </td><td>+ </td><td><a href="/facts/Electron_neutrino/e5hQPWIQ">νe</a> </td><td>+ </td><td>1.20 MeV</td><td>(<a href="/facts/Half-life/kGVH8BAB">half-life</a> of 9.965 minutes<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a>)</td></tr><tr><td>136C </td><td>+ </td><td>11H </td><td>→ </td><td>147N </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>7.54 MeV</td></tr><tr><td>147N </td><td>+ </td><td>11H </td><td>→ </td><td>158O </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>7.35 MeV</td></tr><tr><td>158O </td><td> </td><td> </td><td>→ </td><td>157N </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>1.73 MeV</td><td>(half-life of 122.24 seconds<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a>)</td></tr><tr><td>157N </td><td>+ </td><td>11H </td><td>→ </td><td>126C </td><td>+ </td><td>42He </td><td> </td><td> </td><td>+ </td><td>4.96 MeV</td></tr></tbody></table> <p>where the carbon-12 nucleus used in the first reaction is regenerated in the last reaction. After the two <a href="/facts/Positron_emission/psWp4bJT">positrons emitted</a> <a href="/facts/Annihilation/tm55RWnf">annihilate</a> with two ambient electrons producing an additional 2.04 MeV, the total energy released in one cycle is 26.73 MeV; in some texts, authors are erroneously including the positron annihilation energy in with the <a href="/facts/Beta_decay/vxTBlckp">beta-decay</a> <a href="/facts/Q_value_(nuclear_science)/HwpEN30F">Q-value</a> and then neglecting the equal amount of energy released by annihilation, leading to possible confusion. All values are calculated with reference to the Atomic Mass Evaluation 2003.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> </p><p>The limiting (slowest) reaction in the CNO-I cycle is the <a href="/facts/Proton_capture/eLQDChDd">proton capture</a> on 147N. In 2006 it was experimentally measured down to stellar energies, revising the calculated age of <a href="/facts/Globular_cluster/FL0EERJU">globular clusters</a> by around 1 billion years.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> </p><p>The <a href="/facts/Neutrino/yGWVfSg2">neutrinos</a> emitted in beta decay will have a spectrum of energy ranges, because although <a href="/facts/Conservation_of_momentum/TTkGv5eY">momentum is conserved</a>, the momentum can be shared in any way between the positron and neutrino, with either emitted at rest and the other taking away the full energy, or anything in between, so long as all the energy from the Q-value is used. The total <a href="/facts/Momentum/TTkGv5eY">momentum</a> received by the positron and the neutrino is not great enough to cause a significant recoil of the much <a href="/facts/Invariant_mass/wNGXNBW6">heavier</a> daughter nucleus<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> and hence, its contribution to kinetic energy of the products, for the precision of values given here, can be neglected. Thus the neutrino emitted during the decay of nitrogen-13 can have an energy from zero up to 1.20 MeV, and the neutrino emitted during the decay of oxygen-15 can have an energy from zero up to 1.73 MeV. On average, about 1.7 MeV of the total energy output is taken away by neutrinos for each loop of the cycle, leaving about 25 MeV available for producing <a href="/facts/Luminosity/1tdhpk9l">luminosity</a>.<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> </p> <h3>CNO-II</h3> <p>In a minor branch of the above reaction, occurring in the Sun's core 0.04% of the time, the final reaction involving <a href="/facts/Nitrogen-15/JdTSL0KH">157N</a> shown above does not produce carbon-12 and an alpha particle, but instead produces oxygen-16 and a photon and continues </p> <a href="/facts/Nitrogen-15/JdTSL0KH">157N</a> → <a href="/facts/Oxygen-16/osheCBxR">168O</a> → <a href="/facts/Fluorine-17/nlSCwKKe">179F</a> → <a href="/facts/Oxygen-17/LBY5W95R">178O</a> → <a href="/facts/Nitrogen-14/JdTSL0KH">147N</a> → <a href="/facts/Oxygen-15/ektDXb8B">158O</a> → 157N <p>In detail: </p> <table><tbody><tr><td>157N </td><td>+ </td><td>11H </td><td>→ </td><td>168O </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>12.13 MeV</td></tr><tr><td>168O </td><td>+ </td><td>11H </td><td>→ </td><td>179F </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>0.60 MeV</td></tr><tr><td>179F </td><td> </td><td> </td><td>→ </td><td>178O </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>2.76 MeV</td><td>(half-life of 64.49 seconds)</td></tr><tr><td>178O </td><td>+ </td><td>11H </td><td>→ </td><td>147N </td><td>+ </td><td>42He </td><td> </td><td> </td><td>+ </td><td>1.19 MeV</td></tr><tr><td>147N </td><td>+ </td><td>11H </td><td>→ </td><td>158O </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>7.35 MeV</td></tr><tr><td>158O </td><td> </td><td> </td><td>→ </td><td>157N </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>2.75 MeV</td><td>(half-life of 122.24 seconds)</td></tr></tbody></table> <p>Like the carbon, nitrogen, and oxygen involved in the main branch, the <a href="/facts/Fluorine/YUV0Fb8m">fluorine</a> produced in the minor branch is merely an intermediate product; at steady state, it does not accumulate in the star. </p> <h3>CNO-III</h3> <p>This subdominant branch is significant only for massive stars. The reactions are started when one of the reactions in CNO-II results in fluorine-18 and a photon instead of nitrogen-14 and an alpha particle, and continues </p> <a href="/facts/Oxygen-17/LBY5W95R">178O</a> → <a href="/facts/Fluorine-18/4VpH1NYh">189F</a> → <a href="/facts/Oxygen-18/fK0gerlA">188O</a> → <a href="/facts/Nitrogen-15/JdTSL0KH">157N</a> → <a href="/facts/Oxygen-16/osheCBxR">168O</a> → <a href="/facts/Fluorine-17/nlSCwKKe">179F</a> → 178O <p>In detail: </p> <table><tbody><tr><td>178O </td><td>+ </td><td>11H </td><td>→ </td><td>189F </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>5.61 MeV</td></tr><tr><td>189F </td><td> </td><td> </td><td>→ </td><td>188O </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>1.656 MeV</td><td>(half-life of 109.771 min)</td></tr><tr><td>188O </td><td>+ </td><td>11H </td><td>→ </td><td>157N </td><td>+ </td><td>42He </td><td> </td><td> </td><td>+ </td><td>3.98 MeV</td></tr><tr><td>157N </td><td>+ </td><td>11H </td><td>→ </td><td>168O </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>12.13 MeV</td></tr><tr><td>168O </td><td>+ </td><td>11H </td><td>→ </td><td>179F </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>0.60 MeV</td></tr><tr><td>179F </td><td> </td><td> </td><td>→ </td><td>178O </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>2.76 MeV</td><td>(half-life of 64.49 s)</td></tr></tbody></table> <h3>CNO-IV</h3> <p>Like the CNO-III, this branch is also only significant in massive stars. The reactions are started when one of the reactions in CNO-III results in fluorine-19 and a photon instead of nitrogen-15 and an alpha particle, and continues </p> <a href="/facts/Oxygen-18/fK0gerlA">188O</a> → <a href="/facts/Fluorine-19/nlSCwKKe">199F</a> → <a href="/facts/Oxygen-16/osheCBxR">168O</a> → <a href="/facts/Fluorine-17/nlSCwKKe">179F</a> → <a href="/facts/Oxygen-17/LBY5W95R">178O</a> → <a href="/facts/Fluorine-18/4VpH1NYh">189F</a> → <a href="/facts/Oxygen-18/fK0gerlA">188O</a> <p>In detail: </p> <table><tbody><tr><td>188O </td><td>+ </td><td>11H </td><td>→ </td><td>199F </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>7.994 MeV</td></tr><tr><td>199F </td><td>+ </td><td>11H </td><td>→ </td><td>168O </td><td>+ </td><td>42He </td><td> </td><td> </td><td>+ </td><td>8.114 MeV</td></tr><tr><td>168O </td><td>+ </td><td>11H </td><td>→ </td><td>179F </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>0.60 MeV</td></tr><tr><td>179F </td><td> </td><td> </td><td>→ </td><td>178O </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>2.76 MeV</td><td>(half-life of 64.49 seconds)</td></tr><tr><td>178O </td><td>+ </td><td>11H </td><td>→ </td><td>189F </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>5.61 MeV</td></tr><tr><td>189F </td><td> </td><td> </td><td>→ </td><td>188O </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>1.656 MeV</td><td>(half-life of 109.771 minutes)</td></tr></tbody></table> <p> In some instances 189F can combine with a helium nucleus to start a neon-sodium cycle, in which:<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a><a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> </p><p><a href="/facts/Neon-20/wQHbgept">2010Ne</a> → <a href="/facts/Sodium-21/dGhdEPWX">2111Na</a> → <a href="/facts/Neon-21/wQHbgept">2110Ne</a> → <a href="/facts/Sodium-22/dGhdEPWX">2211Na</a> → <a href="/facts/Neon-22/wQHbgept">2210Ne</a> → <a href="/facts/Sodium-23/dGhdEPWX">2311Na</a> → <a href="/facts/Neon-20/wQHbgept">2010Ne</a> </p><p>The sodium-23 can also turn into magesium-24 after proton bombardment, initiating the magnesium-aluminum cycle. </p> <h2 id="hot-cno-cycles">Hot CNO cycles</h2> <p>Under conditions of higher temperature and pressure, such as those found in <a href="/facts/Nova/H1GcmKXT">novae</a> and <a href="/facts/X-ray_burster/hUAR3j1Q">X-ray bursts</a>, the rate of proton captures exceeds the rate of beta-decay, pushing the burning to the <a href="/facts/Nuclear_drip_line/k3PfJUoy">proton drip line</a>. The essential idea is that a radioactive species will capture a proton before it can beta decay, opening new nuclear burning pathways that are otherwise inaccessible. Because of the higher temperatures involved, these catalytic cycles are typically referred to as the hot CNO cycles; because the timescales are limited by beta decays instead of <a href="/facts/Proton_capture/eLQDChDd">proton captures</a>, they are also called the beta-limited CNO cycles. </p> <h3>HCNO-I</h3> <p>The difference between the CNO-I cycle and the HCNO-I cycle is that <a href="/facts/Nitrogen-13/AMvEc8m7">137N</a> captures a proton instead of decaying, leading to the total sequence </p> <a href="/facts/Carbon-12/nJYECzEr">126C</a>→<a href="/facts/Nitrogen-13/AMvEc8m7">137N</a>→<a href="/facts/Oxygen-14/ektDXb8B">148O</a>→<a href="/facts/Nitrogen-14/JdTSL0KH">147N</a>→<a href="/facts/Oxygen-15/ektDXb8B">158O</a>→<a href="/facts/Nitrogen-15/JdTSL0KH">157N</a>→126C <p>In detail: </p> <table><tbody><tr><td>126C </td><td>+ </td><td>11H </td><td>→ </td><td>137N </td><td>+ </td><td><a href="/facts/Gamma_ray/5Drf913J">γ</a> </td><td> </td><td> </td><td>+ </td><td>1.95 <a href="/facts/Electronvolt/YMT1fqX9">MeV</a></td></tr><tr><td>137N </td><td>+ </td><td>11H </td><td>→ </td><td>148O </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>4.63 MeV</td></tr><tr><td>148O </td><td> </td><td> </td><td>→ </td><td>147N </td><td>+ </td><td><a href="/facts/Positron/HgV4zBzA">e+</a> </td><td>+ </td><td><a href="/facts/Electron_neutrino/e5hQPWIQ">νe</a> </td><td>+ </td><td>5.14 MeV</td><td>(<a href="/facts/Half-life/kGVH8BAB">half-life</a> of 70.641 seconds)</td></tr><tr><td>147N </td><td>+ </td><td>11H </td><td>→ </td><td>158O </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>7.35 MeV</td></tr><tr><td>158O </td><td> </td><td> </td><td>→ </td><td>157N </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>2.75 MeV</td><td>(half-life of 122.24 seconds)</td></tr><tr><td>157N </td><td>+ </td><td>11H </td><td>→ </td><td>126C </td><td>+ </td><td>42He </td><td> </td><td> </td><td>+ </td><td>4.96 MeV</td></tr></tbody></table> <h3>HCNO-II</h3> <p>The notable difference between the CNO-II cycle and the HCNO-II cycle is that <a href="/facts/Fluorine-17/nlSCwKKe">179F</a> captures a proton instead of decaying, and neon is produced in a subsequent reaction on <a href="/facts/Fluorine-18/4VpH1NYh">189F</a>, leading to the total sequence </p> <a href="/facts/Nitrogen-15/JdTSL0KH">157N</a>→<a href="/facts/Oxygen-16/osheCBxR">168O</a>→<a href="/facts/Fluorine-17/nlSCwKKe">179F</a>→<a href="/facts/Neon-18/wQHbgept">1810Ne</a>→<a href="/facts/Fluorine-18/4VpH1NYh">189F</a>→<a href="/facts/Oxygen-15/ektDXb8B">158O</a>→157N <p>In detail: </p> <table><tbody><tr><td>157N </td><td>+ </td><td>11H </td><td>→ </td><td>168O </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>12.13 MeV</td></tr><tr><td>168O </td><td>+ </td><td>11H </td><td>→ </td><td>179F </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>0.60 MeV</td></tr><tr><td>179F </td><td>+ </td><td>11H </td><td>→ </td><td>1810Ne </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>3.92 MeV</td></tr><tr><td>1810Ne </td><td> </td><td> </td><td>→ </td><td>189F </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>4.44 MeV</td><td>(half-life of 1.672 seconds)</td></tr><tr><td>189F </td><td>+ </td><td>11H </td><td>→ </td><td>158O </td><td>+ </td><td>42He </td><td> </td><td> </td><td>+ </td><td>2.88 MeV</td></tr><tr><td>158O </td><td> </td><td> </td><td>→ </td><td>157N </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>2.75 MeV</td><td>(half-life of 122.24 seconds)</td></tr></tbody></table> <h3>HCNO-III</h3> <p>An alternative to the HCNO-II cycle is that <a href="/facts/Fluorine-18/4VpH1NYh">189F</a> captures a proton moving towards higher mass and using the same helium production mechanism as the CNO-IV cycle as </p> 189F→<a href="/facts/Neon-19/wQHbgept">1910Ne</a>→<a href="/facts/Fluorine-19/nlSCwKKe">199F</a>→<a href="/facts/Oxygen-16/osheCBxR">168O</a>→<a href="/facts/Fluorine-17/nlSCwKKe">179F</a>→<a href="/facts/Neon-18/wQHbgept">1810Ne</a>→189F <p>In detail: </p> <table><tbody><tr><td>189F </td><td>+ </td><td>11H </td><td>→ </td><td>1910Ne </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>6.41 MeV</td></tr><tr><td>1910Ne </td><td> </td><td> </td><td>→ </td><td>199F </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>3.32 MeV</td><td>(half-life of 17.22 seconds)</td></tr><tr><td>199F </td><td>+ </td><td>11H </td><td>→ </td><td>168O </td><td>+ </td><td>42He </td><td> </td><td> </td><td>+ </td><td>8.11 MeV</td></tr><tr><td>168O </td><td>+ </td><td>11H </td><td>→ </td><td>179F </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>0.60 MeV</td></tr><tr><td>179F </td><td>+ </td><td>11H </td><td>→ </td><td>1810Ne </td><td>+ </td><td>γ </td><td> </td><td> </td><td>+ </td><td>3.92 MeV</td></tr><tr><td>1810Ne </td><td> </td><td> </td><td>→ </td><td>189F </td><td>+ </td><td>e+ </td><td>+ </td><td>νe </td><td>+ </td><td>4.44 MeV</td><td>(half-life of 1.672 seconds)</td></tr></tbody></table> <h2 id="use-in-astronomy">Use in astronomy</h2> <p>While the total number of "catalytic" nuclei are conserved in the cycle, in <a href="/facts/Stellar_evolution/Qa4vAGmn">stellar evolution</a> 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. During a star's evolution, convective mixing episodes moves material, within which the CNO cycle has operated, from the star's interior to the surface, altering the observed composition of the star. <a href="/facts/Red_giant/lla6Bsvy">Red giant</a> stars are observed to have lower carbon-12/carbon-13 and carbon-12/nitrogen-14 ratios than do <a href="/facts/Main_sequence/DJ3Wq8kd">main sequence</a> stars, which is considered to be convincing evidence for the operation of the CNO cycle.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Aneutronic_fusion/QMVh1qMo">Aneutronic fusion</a></li> <li><a href="/facts/Cold_fusion/UCwaDp88">Cold fusion</a></li> <li><a href="/facts/Fusion_power/jyETSzm4">Fusion power</a></li> <li><a href="/facts/Nuclear_fusion/hP0gLDqj">Nuclear fusion</a></li> <li><a href="/facts/Proton%E2%80%93proton_chain/UUDAR9yW">Proton–proton chain</a>, as found in stars like the Sun</li> <li><a href="/facts/Stellar_nucleosynthesis/qEQJdJe6">Stellar nucleosynthesis</a>, the whole topic</li> <li><a href="/facts/Triple-alpha_process/wV6lacSF">Triple-alpha process</a>, how 12C is produced from lighter nuclei</li></ul> <h2 id="footnotes">Footnotes</h2> <h2 id="further-reading">Further reading</h2> <ul><li>Bethe, H. A. (1939). <a href="https://doi.org/10.1103%2FPhysRev.55.434">"Energy Production in Stars"</a>. <i><a href="/facts/Physical_Review/QxekSF11">Physical Review</a></i>. 55 (5): 434–56. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1939PhRv...55..434B">1939PhRv...55..434B</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1103%2FPhysRev.55.434">10.1103/PhysRev.55.434</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/17835673">17835673</a>.</li> <li>Iben, I. (1967). "Stellar Evolution Within and off the Main Sequence". <i><a href="/facts/Annual_Review_of_Astronomy_and_Astrophysics/DXaYbhdj">Annual Review of Astronomy and Astrophysics</a></i>. 5: 571–626. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1967ARA&A...5..571I">1967ARA&A...5..571I</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1146%2Fannurev.aa.05.090167.003035">10.1146/annurev.aa.05.090167.003035</a>.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Salaris, Maurizio; Cassisi, Santi (2005). Evolution of Stars and Stellar Populations. John Wiley and Sons. pp. 119–121. ISBN 0-470-09220-3. <a href="0-470-09220-3" target="_blank">0-470-09220-3</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Agostini, M.; Altenmüller, K.; et al. (The BOREXINO collaboration) (25 June 2020). "Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun". Nature. 587 (7835): 577–582. arXiv:2006.15115. Bibcode:2020Natur.587..577B. doi:10.1038/s41586-020-2934-0. PMID 33239797. S2CID 227174644. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Reid, I. Neill; Hawley, Suzanne L. (2005). "The structure, formation, and evolution of low-mass stars and brown dwarfs – Energy generation". New Light on Dark Stars: Red dwarfs, low-mass stars, brown dwarfs. Springer-Praxis Books in Astrophysics and Astronomy (2nd ed.). Springer Science & Business Media. pp. 108–111. ISBN 3-540-25124-3. <a href="3-540-25124-3" target="_blank">3-540-25124-3</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Salaris, Maurizio; Cassisi, Santi (2005). Evolution of Stars and Stellar Populations. John Wiley and Sons. pp. 119–121. ISBN 0-470-09220-3. <a href="0-470-09220-3" target="_blank">0-470-09220-3</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Schuler, S.C.; King, J.R.; The, L.-S. (2009). "Stellar Nucleosynthesis in the Hyades open cluster". The Astrophysical Journal. 701 (1): 837–849. arXiv:0906.4812. Bibcode:2009ApJ...701..837S. doi:10.1088/0004-637X/701/1/837. S2CID 10626836. <a href="/wiki/The_Astrophysical_Journal" target="_blank">/wiki/The_Astrophysical_Journal</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>von Weizsäcker, Carl F. (1937). "Über Elementumwandlungen in Innern der Sterne I" [On transformations of elements in the interiors of stars I]. Physikalische Zeitschrift. 38: 176–191. <a href="/wiki/Carl_Friedrich_von_Weizs%C3%A4cker" target="_blank">/wiki/Carl_Friedrich_von_Weizs%C3%A4cker</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>von Weizsäcker, Carl F. (1938). "Über Elementumwandlungen in Innern der Sterne II" [On transformations of elements in the interiors of stars II]. Physikalische Zeitschrift. 39: 633–646. <a href="/wiki/Carl_Friedrich_von_Weizs%C3%A4cker" target="_blank">/wiki/Carl_Friedrich_von_Weizs%C3%A4cker</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Bethe, Hans A. (1939). "Energy Production in Stars". Physical Review. 55 (1): 541–7. Bibcode:1939PhRv...55..103B. doi:10.1103/PhysRev.55.103. PMID 17835673. <a href="/wiki/Hans_Bethe" target="_blank">/wiki/Hans_Bethe</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Bethe, Hans A. (1939). "Energy production in stars". Physical Review. 55 (5): 434–456. Bibcode:1939PhRv...55..434B. doi:10.1103/PhysRev.55.434. PMID 17835673. <a href="/wiki/Hans_Bethe" target="_blank">/wiki/Hans_Bethe</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Agostini, M.; Altenmüller, K.; et al. (The BOREXINO collaboration) (25 June 2020). "Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun". Nature. 587 (7835): 577–582. arXiv:2006.15115. Bibcode:2020Natur.587..577B. doi:10.1038/s41586-020-2934-0. PMID 33239797. S2CID 227174644. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Agostini, M.; Altenmüller, K.; Appel, S.; Atroshchenko, V.; Bagdasarian, Z.; Basilico, D.; Bellini, G.; Benziger, J.; Biondi, R.; Bravo, D.; Caccianiga, B. (25 November 2020). "Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun". Nature. 587 (7835): 577–582. arXiv:2006.15115. Bibcode:2020Natur.587..577B. doi:10.1038/s41586-020-2934-0. ISSN 1476-4687. PMID 33239797. S2CID 227174644. This result therefore paves the way towards a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent; <a href="https://www.nature.com/articles/s41586-020-2934-0" target="_blank">https://www.nature.com/articles/s41586-020-2934-0</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>"Neutrinos yield first experimental evidence of catalyzed fusion dominant in many stars". phys.org. Retrieved 26 November 2020. Pocar points out, "Confirmation of CNO burning in our sun, where it operates at only one percent, reinforces our confidence that we understand how stars work." <a href="https://phys.org/news/2020-11-neutrinos-yield-experimental-evidence-catalyzed.html" target="_blank">https://phys.org/news/2020-11-neutrinos-yield-experimental-evidence-catalyzed.html</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>von Weizsäcker, Carl F. (1937). "Über Elementumwandlungen in Innern der Sterne I" [On transformations of elements in the interiors of stars I]. Physikalische Zeitschrift. 38: 176–191. <a href="/wiki/Carl_Friedrich_von_Weizs%C3%A4cker" target="_blank">/wiki/Carl_Friedrich_von_Weizs%C3%A4cker</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>von Weizsäcker, Carl F. (1938). "Über Elementumwandlungen in Innern der Sterne II" [On transformations of elements in the interiors of stars II]. Physikalische Zeitschrift. 39: 633–646. <a href="/wiki/Carl_Friedrich_von_Weizs%C3%A4cker" target="_blank">/wiki/Carl_Friedrich_von_Weizs%C3%A4cker</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Bethe, Hans A. (1939). "Energy Production in Stars". Physical Review. 55 (1): 541–7. Bibcode:1939PhRv...55..103B. doi:10.1103/PhysRev.55.103. 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