Oxygen burn

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As oxygen burner refers to a group of nuclear fusion reactions in the interior of massive stars with an initial mass of at least eight solar masses , in which by converting oxygen energy is released. It sets in after the lighter elements have been consumed by other fusion processes. The prerequisites for oxygen burning are high temperatures of at least 1.5 · 10 9  Kelvin and high densities of at least 10 10  kg / m 3 .

When burning oxygen, either a) an oxygen nucleus fuses with a helium nucleus to form neon or b) an oxygen nucleus 16 O with a second one to form various new nuclei, including sulfur (S), phosphorus (P), silicon (Si) and magnesium (Mg). In addition, gamma quanta γ, neutrons n, hydrogen nuclei 1 H ( protons ) and alpha particles ( helium nuclei ) 4 He are released:

a)

16 O + 4 He → 20 Ne + γ

However, these Ne nuclei are usually split again when the neon burns .

b)

16 O + 16 O → 32 S + γ
16 O + 16 O → 31 S + n
16 O + 16 O → 31 P + 1 H
16 O + 16 O → 28 Si + 4 He
16 O + 16 O → 24 Mg + 2 4 He

During the previous neon glow , an inactive core of oxygen and magnesium formed in the central area of ​​the star. In the absence of further fuel, the neon burn comes to a standstill. The radiation pressure is no longer sufficient to counteract the gravitation of its own mass, and the core is further compressed. This causes a renewed increase in temperature and density until the ignition temperature for the oxygen burn is reached and the star stabilizes again. Around the core, the so-called shell burning starts again the neon burning; on the outside follow shells with carbon , helium and hydrogen burning .

The oxygen burning lasts only a few years (in the article Stern there is an exemplary time scale for the duration of the individual burning phases). During this time the core becomes enriched with silicon until the oxygen is consumed. The core then cools down again and is compressed by gravity until the last burning stage starts, the silicon burning .

See also

Individual evidence

  1. ↑ N. Langer: Nucleosynthesis . 2012. Retrieved October 19, 2013.
  2. ↑ Clayton, Donald: Principles of Stellar Evolution and Nucleosynthesis , (1983) bibcode : 1983psen.book ..... C .