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.
As the neon-burning process ends, the core of the star contracts and heats until it reaches the ignition temperature for oxygen burning.
In approximately 3 years, the flame's temperature reaches about 1.83 billion kelvins, enabling the oxygen-burning process to commence.
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, two electron capture reactions[clarify] (which produce neutrinos) become significant when the matter density is high enough (ρ > 2×107 g/cm3).
Due to these factors, the timescale of oxygen burning is much shorter for heavy, dense stars.
In the temperature range of (3–4)×109 K, photodisintegration and oxygen fusion occur with comparable reaction rates.
[3] Very massive (140–260 solar masses) population III stars may become unstable during core oxygen burning due to pair production.