Carbon burning

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The carbon-burning process is a set of nuclear fusion reactions by which in massive stars with an initial mass of at least nine solar masses of energy is released. Two carbon nuclei are fused. It occurs after the merger of lighter elements comes to a standstill. The misleading term carbon burning is historical and has nothing to do with chemical combustion .

Carbon burning requires high temperatures of 6 · 10 8  Kelvin - 10 · 10 8  Kelvin and densities of over 10 5  g / cm³. The energy conversion is proportional to the 28th power of the temperature. An increase in temperature by 5% therefore causes an increase to 373% in the energy release.

procedure

The burning of carbon begins when the burning of helium in the core of the star has ceased . The inactive core , which consists mainly of carbon and oxygen, then contracts due to the force of gravity , which causes an increase in temperature and density until the ignition temperature for the carbon burning is finally reached. The radiation pressure then generated stabilizes the core and its further compression is stopped. The fusion of two carbon nuclei can produce different nuclei:

12 C + 12 C 24 Mg + γ  
12 C + 12 C 23 Mg + n ( endothermic )
12 C + 12 C 23 Na + 1 H  
12 C + 12 C 20 Ne + 4 He  
12 C + 12 C 16 O + 2 4 He (endothermic)

For the two reactions marked as endothermic , energy has to be expended; i.e., they do not provide energy to the star. The light particles released ( protons , alpha particles, etc.) cause a large number of secondary reactions both with the products of carbon burning and with 12 C and 16 O nuclei. On average, 10 MeV of energy is released per fusion of two 12 C nuclei . The second reaction, in which magnesium 23 Mg and one neutron n are formed, is one of the few fusion processes in the course of star evolution in which neutrons are released at all.

During carbon burning, the core area is enriched with the reaction products oxygen, magnesium (Mg) and neon (Ne), until after a few thousand years the carbon is used up and the fusion reaction comes to a standstill. The core then cools down again and contracts again.

For stars with an initial mass between nine and 11 solar masses, carbon burning is the final thermonuclear burning process. In their further development they form a planetary nebula . A white dwarf, consisting primarily of oxygen and neon and about 1.2 solar masses, arises from its core . Stars with larger masses can also ignite the following burning processes in their core, starting with the burning of neon . The duration of carbon burning also depends on the star's initial mass: Stars with 10 solar masses need about 20,000 years for carbon burning in their core, stars with 25 solar masses only need 1,600 years.

See also

Individual evidence

  1. ^ A b c Christian Iliadis: Nuclear Physics of Stars . 2nd Edition. Wiley-VCH, Weinheim 2015, ISBN 978-3-527-33648-7 , pp. 22 (English).
  2. ^ A b c d Christian Iliadis: Nuclear Physics of Stars . 2nd Edition. Wiley-VCH, Weinheim 2015, ISBN 978-3-527-33648-7 , 5.3.1 Carbon Burning, p. 400-407 (English).