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
The neon-burning process is a set of nuclear fusion reactions that take place in evolved massive stars with at least 8 Solar masses. Neon burning requires high temperatures and densities (around 1.2×109 K or 100 keV and 4×109 kg/m3).
At such h ...
and succeeded by the
silicon-burning process
In astrophysics, silicon burning is a very brief sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8–11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the f ...
. 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
9 K
[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.] and in the density range of (2.6–6.7)×10
12 kg·m
−3.
[Hirschi. "Evolution and nucleosynthesis of Very Massive Stars". arXiv:1409.7053v1 stro-ph.SR24 Sep 2014.] The principal reactions are given below,
[Woosley, Heger, and Weaver. "The evolution of massive stars". Reviews of Modern Physics, Volume 74, October 2002.][Clayton, Donald. Principles of Stellar Evolution and Nucleosynthesis, (1983).] where the branching ratios assume that the
deuteron channel is open (at high temperatures):
:
Near 2×10
9 K, the oxygen-burning reaction rate is approximately 2.8×10
−12(''T''
9/2)
33,
where ''T''
9 is the temperature in billion
kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...
s. Overall, the major products of the oxygen-burning process are
28Si,
32,33,34S,
35,37Cl,
36,38Ar,
39,41K, and
40,42Ca. Of these,
28Si and
32S constitute 90% of the final composition.
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.
The
silicon-burning process
In astrophysics, silicon burning is a very brief sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8–11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the f ...
, 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
16O is lighter than neon, neon burning occurs before oxygen burning, because
16O is a
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
16O +
16O.
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 (
4He(2α,γ)
12C(α,γ)
16O), carbon-burning process (
12C(
12C,α)
20Ne,
12C(α,γ)
16O), and neon-burning process (
20Ne(γ,α)
16O). The reaction
12C(α,γ)
16O has a significant effect on the reaction rates during oxygen burning, as it produces large quantities of
16O.
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
convective
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convect ...
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 around 10
36 W. After only a month, the power declines to about 10
35 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 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 years, the flame's temperature reaches about 1.83 billion kelvins, enabling the oxygen-burning process to commence. This occurs around 9.5 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.
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, (which produce neutrinos) become significant when the matter density is high enough (ρ > 2×10
7 g/cm
3). Due to these factors, the timescale of oxygen burning is much shorter for heavy, dense stars.
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
9 K, photodisintegration and oxygen fusion occur with comparable reaction rates.
Pair-instability supernovae
Very massive (140–260 solar masses)
population III stars may become unstable during core oxygen burning due to
pair production
Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production often refers specific ...
. This results in a thermonuclear explosion, which completely disrupts the star.
References
External links
Fusion of Carbon and Oxygen/ The Astrophysics spectator, 2005
* Arnett, W. D
Advanced evolution of massive stars. VI – Oxygen burning/ Astrophysical Journal, vol. 194, Dec. 1, 1974, pt. 1, p. 373–383.
{{Star
Nucleosynthesis