Energy generation in stars
In the early twentieth century science was wondering what was the source of the incredible energy that fed the stars. None of the known solutions in feasible time. No chemical reaction yields necessary to keep off the sun light that also the gravitational contraction, although it was a source of energy, could not explain the contribution of heat over billions of years. Sir Arthur Eddington first suggested in the 1920s that the contribution of energy came from nuclear reactions. There are two types of nuclear reactions, the fission and fusion. The fission reactions can not maintain the brightness of a star due to its relatively low energy efficiency and, especially, require that elements heavier than iron, which are not abundant in the universe. The first detailed mechanism of nuclear fusion reactions are capable of maintaining the internal structure of a star was discovered by Hans Bethe in 1938, is valid for stars of intermediate or high mass and is named after its discoverer (or Bethe cycle CNO cycle) .
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A variety of different fusion reactions take place inside the cores of stars, which depend on the mass and composition.
Normally, the stars begin burning their nuclear about 75 hydrogen and 25 helium with small traces of other elements. At the core of the Sun with about 107 K the hydrogen is fused into helium through the proton-proton chain:
4A H ‘2 2e 2’e H (4.0 MeV 1.0 MeV)
2 2 H H ‘ 2 I 2’ (5.5 MeV)
He 2 ‘ 2 H 4HE (12.9 MeV)
These reactions are reduced in the overall reaction:
4A H ‘4HE 2e 2’ 2’e (26.7 MeV)
In more massive helium stars are produced in a cycle of reactions catalyzed by carbon, or CNO cycle is the cycle Bethe. This is represented in the exemplary case of a star with 18 solar masses:
The stars whose nuclei are at 108 K and whose masses range from 0.5 to 10 solar masses of helium resulting from the initial reactions can be transformed into carbon through the triple-alpha process:
4HE 4HE 92 keV ‘Be 8
Be 4HE 8 keV 67 ’12 C
12 C ’12C’ 7.4 MeV
The overall reaction is:
34He ’12C’ 7.2 MeV
See also: stellar nucleosynthesis, Gamow peak, and Evolution