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Concept
Fusion de l'oxygène
Résumé
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. Oxygen-burning is preceded by the neon-burning process and succeeded by the silicon-burning process. As the neon-burning process ends, the core of the star contracts and heats until it reaches the ignition temperature for oxygen burning. Oxygen burning reactions are similar to those of carbon burning; however, they must occur at higher temperatures and densities due to the larger Coulomb barrier of oxygen.
Oxygen ignites in the temperature range of (1.5–2.6)×109 K and in the density range of (2.6–6.7)×1012 kg·m−3. The principal reactions are given below, where the branching ratios assume that the deuteron channel is open (at high temperatures):
{| border="0"
|- style="height:2em;"
| ||+ || ||→ || ||+ || ||+ ||9.593 MeV (34%)
|- style="height:2em;"
| || || ||→ || ||+ || ||+ ||7.676 MeV (56%)
|- style="height:2em;"
| || || ||→ || ||+ ||_Neutron ||+ ||1.459 MeV (5%)
|- style="height:2em;"
| || || ||→ || ||+ ||2 ||+ ||0.381 MeV
|- style="height:2em;"
| || || ||→ || ||+ || ||− ||2.409 MeV (5%)
|- style="height:2em;"
|colspan=99|Alternatively:
|- style="height:2em;"
| || || ||→ || ||+ ||_Gamma
| +
|16.539 MeV
|- style="height:2em;"
| || || ||→ || ||+ ||2
| −
|0.393 MeV
|}
Near 2×109 K, the oxygen-burning reaction rate is approximately 2.8×10−12(T9/2)33, where T9 is the temperature in billion kelvins. Overall, the major products of the oxygen-burning process are 28Si, 32,33,34S, 35,37Cl, 36,38Ar, 39,41K, and 40,42Ca. Of these, 28Si and 32S constitute 90% of the final composition. The oxygen fuel within the core of the star is exhausted after 0.01–5 years, depending on the star's mass and other parameters. The silicon-burning process, which follows, creates iron, but this iron cannot react further to create energy to support the star.
During the oxygen-burning process, proceeding outward, there is an oxygen-burning shell, followed by a neon shell, a carbon shell, a helium shell, and a hydrogen shell.
Source officielle
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