Carbon burning process
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The carbon burning process is a nuclear fusion reaction that occurs in massive stars (at least 4 MSun at birth) that have used up the lighter elements in their cores. It requires high temperatures (6×108 K) and densities (about 2×108 kg/m3</sub>).
12C + 12C → 24Mg + γ → 23Mg + n → 23Na + 1H → 20Ne + 4He → 16O + 24He Carbon burning starts when Helium burning ends. During helium burning, stars build up an inert core rich in carbon and oxygen. Once helium is consumed in the core, it collapses due to gravitation. This decrease in volume raises temperature and density of the core up to the carbon ignition temperature. This will raise the star's temperature around the core allowing it to burn helium in a shell around the core. The star increases in size and becomes a red supergiant.
As carbon burns, reaction products (O, Mg, Ne) accumulate in a new inert core. After some one thousand years the star will run out of carbon, and the core cools down again and contracts. This heats up the core up to the Neon ignition (see Neon burning process). Around the core carbon continues to burn in a shell, and continuing outwards there is a helium burning shell and a hydrogen burning shell.
At this point, stars with masses between 4 and 8 solar masses destabilize and eject the envelope in a massive stellar wind, leaving behind a O-Ne-Mg white dwarf core.
Stars with bigger masses proceed with Neon burning process, but the evolution from now on is so quick that the envelope usually can no longer respond.