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Oxygen burning is fusion of two oxygen nuclei, which takes place in early stars, and produces silicon, phosphorus, sulfur, and magnesium. An oxygen-burning shell appears in a sufficiently-massive star during a phase in the asymptotic giant branch, forming after the neon-burning shell and before the silicon-burning shell (if the latter forms). Oxygen burns after neon because it requires a higher temperature (on the order of 109 K): an oxygen nucleus is more stable and more kinetic energy is required to bring the nuclei together.