Bethe-Weizsäcker cycle

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The CNO cycle.

The Bethe-Weizsäcker cycle (also CN cycle , CNO cycle , CNO-I cycle , carbon-nitrogen cycle ) is one of the eight fusion reactions of the so-called hydrogen combustion , through which stars convert hydrogen into helium ; the others are the proton-proton reaction and other possible CNO cycles , which, however, take place at even higher temperatures.

The cycle was discovered between 1937 and 1939 by the physicists Hans Bethe and Carl Friedrich von Weizsäcker . The names CN or CNO cycle are derived from the elements carbon (C), nitrogen (N) and oxygen (O) involved in the reaction . While the proton-proton reaction plays a more important role in stars with sizes up to the mass of the Sun , theoretical models show that the Bethe-Weizsäcker cycle is believed to be the predominant source of energy in heavier stars. The sun itself generates only 1.6% of its energy through the Bethe-Weizsäcker cycle.

The Bethe-Weizsäcker cycle only takes place at temperatures above 14 million Kelvin and is predominant from 30 million Kelvin. The conversion rate is proportional to the amount of 12 C.

Since, according to current opinion, no carbon could be produced during the Big Bang , it was impossible for the stars of the first generation ( Population III ) to generate energy in this way. In the late phases of star development, however, carbon is created in the stars through the three-alpha process (see also nucleosynthesis ), which is then available on the one hand as a catalyst and on the other hand is released through supernovae to the interstellar medium, from which new stars are formed form.

Stars of later generations therefore contain carbon at the beginning of their development (see also metallicity ).

In the Bethe-Weizsäcker cycle, fusions of hydrogen nuclei 1 H ( protons ) with the heavier nuclei 12 C, 13 C, 14 N and 15 N essentially take place , hence the name CN cycle . During fusion, some energy is released in the form of gamma quanta  γ. Two of the intermediate products formed, 13 N and 15 O, are unstable and decay after a short time, each emitting a positron  e + and an electron  neutrino ν e . The individual reaction steps are listed below.

        Average response time (in years)
12 C + 1 H → 13 N + γ + 1.95  MeV 1.3 · 10 7
13 N. → 13 C + e + + ν e + 1.37 MeV 1.3 · 10 −5
13 C + 1 H. → 14 N + γ + 7.54 MeV 2.6 · 10 6
14 N + 1 H. → 15 O + γ + 7.35 MeV 3.2 · 10 8
15 O → 15 N + e + + ν e + 1.86 MeV 2.6 · 10 −6
15 N + 1 H. → 12 C + 4 He + 4.96 MeV 1.1 · 10 5

The overall result of the cycle is the fusion of four hydrogen nuclei 1 H to form a helium nucleus 4 He, the mass of which is almost 1 percent less than the mass of the four protons ( mass defect ). The difference is converted into energy and neutrinos according to the Einstein equation E  =  mc ². The energy balance here is +25.03 MeV. The carbon core 12 C only serves as a catalyst and is finally regenerated with the last reaction. The energy that the neutrinos carry in the form of their low mass and, above all, their kinetic energy , is withdrawn from the star, as they can escape almost unhindered through the stellar matter.

A complete cycle through the cycle takes enormous amounts of time - on the order of hundreds of millions of years for massive stars. In massive stars, the cycle is faster than the proton-proton reaction (several billion years), so stars can release significantly more energy in this way.

In the Bethe-Weizsäcker cycle, the rate of energy generation is proportional to the 18th power of the temperature. An increase in temperature by 5% therefore increases the energy release by approx. 141%.

The "ash" of the hydrogen burning is helium 4 He, which can serve as a starting material for the helium burning that may start later .

See also

literature

Individual evidence

  1. ↑ Eric G. Adelberger et al .: Solar fusion cross sections. II. The pp chain and CNO cycles . In: Reviews of Modern Physics . tape 83 , no. 1 , 2011, p. 226 , doi : 10.1103 / RevModPhys.83.195 .