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The carbon burning process is a set of nuclear fusion reactions that take place 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). The principal reactions are:
| 126C | + | 126C | → | 2010Ne | + | 42He | + | 4.617 MeV | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 126C | + | 126C | → | 2311Na | + | 1H | + | 2.241 MeV | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 126C | + | 126C | → | 2312Mg | + | n0 | - | 2.599 MeV | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Alternatively: | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 126C | + | 126C | → | 2412Mg | + | γ | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 126C | + | 126C | → | 168O | + | 2 42He | (endothermic) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Carbon burning starts when helium burning ends. During helium fusion, stars build up an inert core rich in carbon and oxygen. Once the helium density drops below a level at which He burning can be sustained, the core 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 time (perhaps ~one thousand years[citation needed]) the carbon density will drop below a level at which C-burning can be sustained, and the core cools down again and contracts. This heats 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 an O-Ne-Mg white dwarf core.
Stars with bigger masses proceed with the neon burning process, but the evolution from now on is so quick that the envelope usually can no longer respond.
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