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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. It occurs at temperatures around 1.5×109K / 130 keV and densities of 1010 kg/m3. The principal reactions are:1
With the neon-burning process an inert core of O-Mg forms in the centre of the star. As the neon burning turns off, the core contracts and heats up to the ignition point for the oxygen burning. In about six months to one year the star consumes its oxygen, accumulating a new core rich in silicon. This core is inert because it is not hot enough for silicon burning. Once oxygen is exhausted, the core ceases producing fusion energy and contracts. This contraction heats it up to the point that the silicon-burning process ignites. Proceeding outward, there is an oxygen-burning shell, followed by the neon shell, the carbon shell, the helium shell, and the hydrogen shell.
The oxygen-burning process is the last nuclear reaction in the star's core which does not proceed via the alpha process.