The carbon-burning process or carbon fusion is a set of nuclear fusion reactions that take place in the cores of massive stars (at least 8 at birth) that combines carbon into other elements. It requires high temperatures (> 5×108 K or 50 keV) and densities (> 3×109 kg/m3).
These figures for temperature and density are only a guide. More massive stars burn their nuclear fuel more quickly, since they have to offset greater gravitational forces to stay in (approximate) hydrostatic equilibrium. That generally means higher temperatures, although lower densities, than for less massive stars. To get the right figures for a particular mass, and a particular stage of evolution, it is necessary to use a numerical stellar model computed with computer algorithms. Such models are continually being refined based on nuclear physics experiments (which measure nuclear reaction rates) and astronomical observations (which include direct observation of mass loss, detection of nuclear products from spectrum observations after convection zones develop from the surface to fusion-burning regions – known as dredge-up events – and so bring nuclear products to the surface, and many other observations relevant to models).
Fusion reactions
The principal reactions are: :{ | border="0" |- style="height:3em;" || ||+ || ||→ || ||+ || ||+ ||4.617 MeV |- style="height:3em;" | ||+ || ||→ || ||+ || ||+ ||2.241 MeV |- style="height:3em;" | ||+ || ||→ || ||+ ||1n ||− ||2.599 MeV |- style="height:3em;" |colspan=99|Alternatively: |- style="height:3em;" | ||+ || ||→ || ||+ || ||+ ||13.933 MeV |- style="height:3em;" | ||+ || ||→ || ||+ ||2 ||colspan=2|− 0.113 MeV |}
Reaction products
This sequence of reactions can be understood by thinking of the two interacting carbon nuclei as coming together to form an excited state of the 24Mg nucleus, which then decays in one of the five ways listed above. The first two reactions are strongly exothermic, as indicated by the large positive energies released, and are the most frequent results of the interaction. The third reaction is strongly endothermic, as indicated by the large negative energy indicating that energy is absorbed rather than emitted. This makes it much less likely, yet still possible in the high-energy environment of carbon burning. But the production of a few neutrons by this reaction is important, since these neutrons can combine with heavy nuclei, present in tiny amounts in most stars, to form even heavier isotopes in the s-process.
The fourth reaction might be expected to be the most common from its large energy release, but in fact it is extremely improbable because it proceeds via electromagnetic interaction, as it produces a gamma ray photon, rather than utilising the strong force between nucleons as do the first two reactions. Nucleons look a lot bigger to each other than they do to photons of this energy. However, the 24Mg produced in this reaction is the only magnesium left in the core when the carbon-burning process ends, as 23Mg is radioactive.
The last reaction is also very unlikely since it involves three reaction products, as well as being endothermic — think of the reaction proceeding in reverse, it would require the three products all to converge at the same time, which is less likely than two-body interactions.
The protons produced by the second reaction can take part in the proton–proton chain reaction, or the CNO cycle, but they can also be captured by 23Na to form 20Ne plus a 4He nucleus. In fact, a significant fraction of the 23Na produced by the second reaction gets used up this way. In stars between 9 and 11 solar masses, the 16O already produced by helium fusion in the previous stage of stellar evolution manages to survive the carbon-burning process pretty well, despite some of it being used up by capturing 4He nuclei. So the end result of carbon burning is a mixture mainly of oxygen, neon, sodium and magnesium.
The fact that the mass-energy sum of the two carbon nuclei is similar to that of an excited state of the magnesium nucleus is known as 'resonance'. Without this resonance, carbon burning would only occur at temperatures one hundred times higher. The experimental and theoretical investigation of such resonances is still a subject of research. A similar resonance increases the probability of the triple-alpha process, which is responsible for the original production of carbon.