View source for Oxygen-burning process

{{for|the chemical sense of burning with oxygen|combustion}}
{{Short description|Set of nuclear fusion reactions}}
The '''oxygen-burning process''' is a set of [[nuclear fusion]] reactions that take place in massive stars that have used up the lighter elements in their cores. Oxygen-burning is preceded by the [[neon-burning process]] and succeeded by the [[silicon-burning process]]. As the neon-burning process ends, the core of the star contracts and heats until it reaches the ignition temperature for oxygen burning.  Oxygen burning reactions are similar to those of carbon burning; however, they must occur at higher temperatures and densities due to the larger [[Coulomb barrier]] of oxygen.

==Reactions==
Oxygen ignites in the temperature range of (1.5–2.6)×10<sup>9</sup>&nbsp;K<ref name=":1">El Eid, M. F., B. S. Meyer, and L.‐S. The. "Evolution of Massive Stars Up to the End of Central Oxygen Burning." ApJ The Astrophysical Journal 611.1 (2004): 452–65. Arxiv.org. 21 July 2004. Web. 8 Apr. 2016.</ref> and in the density range of (2.6–6.7)×10<sup>12</sup>&nbsp;kg·m<sup>−3</sup>.<ref name=":6">Hirschi. "Evolution and nucleosynthesis of Very Massive Stars". arXiv:1409.7053v1 [astro-ph.SR] 24 Sep 2014.</ref> The principal reactions are given below,<ref name=":0">Woosley, Heger, and Weaver. "The evolution of massive stars". Reviews of Modern Physics, Volume 74, October 2002.</ref><ref>Clayton, Donald. Principles of Stellar Evolution and Nucleosynthesis, (1983).</ref> where the branching ratios assume that the [[deuteron]] channel is open (at high temperatures):<ref name=":0" /><!-- Autogenerated using Phykiformulae 0.12 [[User:SkyLined#Phykiformulae]]
	 + O-16 -> Si-28 + He + 9.594MeV 
	_    _ _    -> P-31  + H  + 7.678MeV 
	_    _ _    -> S-31  + n  + 1.500MeV 
	_    _ _    -> Si-30 + 2H + 0.381MeV 
	_    _ _    -> P-30  + H  − 2.409MeV
	!Alternatively:
	_    _ _    -> S-32  + y
	_    _ _    -> Mg-24 + 2He
-->
:{| border="0"
|- style="height:2em;"
|{{nuclide|link=yes|oxygen|16}}&nbsp;||+&nbsp;||{{nuclide|link=yes|oxygen|16}}&nbsp;||→&nbsp;||{{nuclide|link=yes|silicon|28}}&nbsp;||+&nbsp;||[[Alpha particle|{{nuclide|helium|4}}]]&nbsp;||+&nbsp;||9.593&nbsp;[[electron volt|MeV]]&nbsp;(34%)
|- style="height:2em;"
|&nbsp;||&nbsp;||&nbsp;||→&nbsp;||{{nuclide|link=yes|phosphorus|31}}&nbsp;||+&nbsp;||{{nuclide|link=yes|hydrogen|1}}&nbsp;||+&nbsp;||7.676&nbsp;[[electron volt|MeV]]&nbsp;(56%)
|- style="height:2em;"
|&nbsp;||&nbsp;||&nbsp;||→&nbsp;||{{nuclide|link=yes|sulfur|31}}&nbsp;||+&nbsp;||{{SubatomicParticle|link=yes|Neutron}}&nbsp;||+&nbsp;||1.459&nbsp;[[electron volt|MeV]] (5%)
|- style="height:2em;"
|&nbsp;||&nbsp;||&nbsp;||→&nbsp;||{{nuclide|link=yes|silicon|30}}&nbsp;||+&nbsp;||2&nbsp;{{nuclide|link=yes|hydrogen|1}}&nbsp;||+&nbsp;||0.381&nbsp;[[electron volt|MeV]]
|- style="height:2em;"
|&nbsp;||&nbsp;||&nbsp;||→&nbsp;||{{nuclide|link=yes|phosphorus|30}}&nbsp;||+&nbsp;||[[Deuterium|{{nuclide|deuterium}}]]&nbsp;||&minus;&nbsp;||2.409&nbsp;[[electron volt|MeV]] (5%)
|- style="height:2em;"
|colspan=99|Alternatively:<ref name=":2">Caughlan and Fowler. "Thermonuclear reaction rates”. Atomic Data and Nuclear Data Tables, 40, 283–334 (1988).</ref><ref name=":7">Kasen, Woosley, and Heger. "Pair Instability Supernovae: Light Curves, Spectra, and Shock Breakout". The Astrophysical Journal 734:102, 2011 June 20.</ref><ref name=":5">Carroll, Bradley W., and Dale A. Ostlie. "An Introduction to Modern Astrophysics". San Francisco, Pearson Addison-Wesley, 2007.</ref><ref name=":3">S. E. Woosley and Alexander Heger. "The Remarkable Deaths of 9–10 Solar Mass Stars". arXiv:1505.06712v1. May 2015.</ref><ref name=":4">Longair, Malcolm. "High Energy Astrophysics", 3rd edition, (2011).</ref>
|- style="height:2em;"
|&nbsp;||&nbsp;||&nbsp;||→&nbsp;||{{nuclide|link=yes|sulfur|32}}&nbsp;||+&nbsp;||{{SubatomicParticle|link=yes|Gamma}}
| +
|16.539 MeV
|- style="height:2em;"
|&nbsp;||&nbsp;||&nbsp;||→&nbsp;||{{nuclide|link=yes|magnesium|24}}&nbsp;||+&nbsp;||2&nbsp;[[Alpha particle|{{nuclide|helium|4}}]]
| &minus;
|0.393 MeV
|}

Near 2×10<sup>9</sup>&nbsp;K, the oxygen-burning reaction rate is approximately 2.8×10<sup>−12</sup>(''T''<sub>9</sub>/2)<sup>33</sup>{{clarify|reason=units?|date=December 2019}},<ref name=":0" /><ref name=":2" /> where ''T''<sub>9</sub> is the temperature in billion [[kelvin]]s<!-- lower-case and pluralized! -->. Overall, the major products of the oxygen-burning process are <sup>28</sup>Si, <sup>32,33,34</sup>S, <sup>35,37</sup>Cl, <sup>36,38</sup>Ar, <sup>39,41</sup>K, and <sup>40,42</sup>Ca. Of these, <sup>28</sup>Si and <sup>32</sup>S constitute 90% of the final composition.<ref name=":0" /> The oxygen fuel within the core of the star is exhausted after 0.01–5 years, depending on the star's mass and other parameters.<ref name=":1" /><ref name=":0" /> The [[silicon-burning process]], which follows, creates iron, but this iron cannot react further to create energy to support the star.

During the oxygen-burning process, proceeding outward, there is an oxygen-burning shell, followed by a neon shell, a carbon shell, a helium shell, and a hydrogen shell. The oxygen-burning process is the last nuclear reaction in the star's core which does not proceed via the [[alpha process]].

== Pre-oxygen burning ==
Although <sup>16</sup>O is lighter than neon, neon burning occurs before oxygen burning, because <sup>16</sup>O is a [[Magic number (physics)|doubly-magic]] nucleus and hence extremely stable. Compared to oxygen, neon is much less stable. As a result, neon burning occurs at lower temperatures than <sup>16</sup>O&nbsp;+&nbsp;<sup>16</sup>O.<ref name=":4" /> During neon burning, oxygen and magnesium accumulate in the core of the star. At the onset of oxygen burning, oxygen in the stellar core is plentiful due to the helium-burning process (<sup>4</sup>He(2α,γ)<sup>12</sup>C(α,γ)<sup>16</sup>O), carbon-burning process (<sup>12</sup>C(<sup>12</sup>C,α)<sup>20</sup>Ne, <sup>12</sup>C(α,γ)<sup>16</sup>O), and neon-burning process (<sup>20</sup>Ne(γ,α)<sup>16</sup>O). The reaction <sup>12</sup>C(α,γ)<sup>16</sup>O has a significant effect on the reaction rates during oxygen burning, as it produces large quantities of <sup>16</sup>O.<ref name=":0" />

== Convectively bounded flames and off-center oxygen ignition ==
For stars with masses greater than 10.3 solar masses, oxygen ignites in the core or not at all. Similarly, for stars with a mass of less than 9 solar masses (without accretion of additional mass) oxygen ignites in the core or not at all. However, in the 9–10.3 solar mass range, oxygen ignites off-center.

For stars in this mass range neon-burning occurs in a [[Convection|convective]] envelope rather than at the core of the star. For the particular example of a 9.5 solar mass star, the neon-burning process takes place in an envelope of approximately 0.252 solar masses (~1560 kilometers) off center. From the ignition flash, the neon convective zone extends further out to 1.1 solar masses with a peak [[Power (physics)|power]] around 10<sup>36</sup>&nbsp;W.  After only a month, the power declines to about 10<sup>35</sup>&nbsp;W and stays at this rate for about 10 years.  After this phase, the neon in the shell is depleted, resulting in greater inward pressure on the star.  This raises the shell's temperature to 1.65&nbsp;billion kelvins.  This results in a neon-burning, convectively-bound flame front that moves toward the core.  The motion of the flame is what eventually leads to oxygen-burning.  In approximately 3&nbsp;years, the flame's temperature reaches about 1.83&nbsp;billion kelvins, enabling the oxygen-burning process to commence. This occurs around 9.5&nbsp;years before the iron core develops. Similarly to the beginning of neon-burning, off-center oxygen-burning commences with another flash.  The convectively burning flame then results from both neon and oxygen burning as it advances towards the core, while the oxygen-burning shell continuously shrinks in mass.<ref name=":3" />

== Neutrino losses ==
During the oxygen-burning process, energy loss due to neutrino emission becomes relevant. Due to the large energy loss, oxygen must burn at temperatures higher than a billion kelvins in order to maintain a radiation pressure strong enough to support the star against gravity. Further, {{clarify span|two electron capture reactions|"two ... reaction" (no hyphen) or "two-electron ... reactions" (hyphenated)? In any case, need to show these reactions|date=December 2019}} (which produce neutrinos) become significant when the matter density is high enough (ρ&nbsp;>&nbsp;2×10<sup>7</sup>&nbsp;g/cm<sup>3</sup>). Due to these factors, the timescale of oxygen burning is much shorter for heavy, dense stars.<ref name=":5" />

== Explosive oxygen burning ==
The oxygen-burning process can occur under hydrostatic and under explosive conditions. The products of explosive oxygen burning are similar to those in hydrostatic oxygen burning. However, stable oxygen burning is accompanied by a multitude of electron captures, while explosive oxygen burning is accompanied by a significantly greater presence of [[photodisintegration]] reactions. In the temperature range of (3–4)×10<sup>9</sup> K, photodisintegration and oxygen fusion occur with comparable reaction rates.<ref name=":0" />

== Pair-instability supernovae ==
{{main|Pair-instability supernova}}

Very massive (140–260 solar masses) [[Population III star|population III]] stars may become unstable during core oxygen burning due to [[pair production]]. This results in a thermonuclear explosion, which completely disrupts the star.<ref name=":6" /><ref name=":7" />

==References==
{{Reflist}}

==External links==
* [http://www.astrophysicsspectator.com/topics/stars/FusionCarbonOxygen.html Fusion of Carbon and Oxygen] / The Astrophysics spectator, 2005
* Arnett, W. D. [http://adsabs.harvard.edu/full/1974ApJ...194..373A  Advanced evolution of massive stars. VI – Oxygen burning] / Astrophysical Journal, vol. 194, Dec. 1, 1974, pt. 1, p.&nbsp;373–383.

{{Nuclear_processes}}
{{Star}}

[[Category:Nucleosynthesis]]