Oxygen burning process
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. Oxygen in the core ignites in the temperature range of (1.5-2.6)×109 K and in the density range of (2.6-6.7)×109g/cm3. The principal reactions are given below, where the branching ratios assume that the deuteron channel is open (at high temperatures):
Near 2×109K, the oxygen burning reaction rate is approximately 2.8×10−12(T9/2)33, where T9 is the temperature in billions of degrees Kelvin. 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 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.
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.
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