The Astrophysics Spectator
The Astrophysics Spectator
Following the complete burning of helium-4 into carbon, oxygen, and other elements within the core of a star, the core begins to collapse again until the next fusion stage is reached: the burning of carbon into heavier elements. This stage is then followed by the fourth stage of thermonuclear fusion, the burning of oxygen into heavier elements. Each of these stages is much more complex than either the hydrogen or the helium burning stages, because the number of fusion processes and the variety of fusion products are much richer than in the fusion of hydrogen and helium. Among the products are protons, which burn further through the CNO hydrogen fusion process, neutrons, which can combine with atomic nuclei to produce heavier isotopes, and helium-4, which can burn through one of the processes detailed on the helium fusion page. On this page, only the most important processes are presented.
In the carbon-fusion stage, two carbon-12 nuclei fuse to create heavier elements. Carbon preferentially interacts only with itself, unlike helium, which interacts with heavy elements such as carbon-12 and oxygen-16. In particular, there is no appreciable interaction between carbon-12 and oxygen-16. The primary nuclei created through carbon fusion are sodium-23 (Na23) and neon-20 (Ne20).
Carbon fusion begins at about 600 to 700 million degrees (50 to 60 keV). The most energetic carbon-carbon reaction liberates approximately 13 MeV of energy as magnesium-24 (Mg24) is created. Other carbon-carbon reactions liberate considerably less energy than this, and in some cases consuming energy. Much of this energy escapes from the star as neutrinos, even though none of the principle carbon fusion reactions emit neutrinos. The principal reactions are as follows:
| C12 + C12 | → | Mg24 + |
| C12 + C12 | → | Na23 + p |
| C12 + C12 | → | Ne20 + He4 |
| C12 + C12 | → | Mg23 + n |
| C12 + C12 | → | O16 + 2 He4 |
Of these processes, the first-three are exothermic, releasing 13.93 MeV, 2.24 MeV, and 4.62 MeV respectively, and the last-two are endothermic, absorbing 2.60 MeV and 0.11 MeV respectively of energy.
In oxygen fusion, two oxygen nuclei fuse to create elements with atomic mass at or below the mass of sulfur-32. Many different nuclei are created in this process, although silicon-28 (Si28) is the the major product from the nuclear fusion of oxygen.
Oxygen fusion begins at about 1 billion degrees (90 keV). The energy released is more uncertain than for the carbon burning, but it is comparable in value. Neutrino production is so great for oxygen fusion that most of the energy liberated is transported out of the core by the neutrinos, so only a small part of the energy release in oxygen fusion is available to replace energy that is radiatively transported out of the star.
The principal oxygen fusion reactions are as follows:
| O16 + O16 | → | S32 + |
| O16 + O16 | → | P31 + p |
| O16 + O16 | → | S31 + n |
| O16 + O16 | → | Si28 + He4 |
| O16 + O16 | → | Mg24 + 2 He4 |
The first-four reactions are exothermic, releasing 16.54 MeV, 7.68 MeV, 1.46 MeV, and 9.59 MeV respectively. The last reaction is endothermic, absorbing 0.39 MeV of energy.
