CNO cycle

The Bethe- Weizsäcker - cycle (also CNO cycle, CN - cycle carbon-nitrogen cycle ) is one of two fusion reactions of the so-called hydrogen burning, converting hydrogen into helium through the stars; the other is the proton-proton reaction.

The cycle was discovered in 1937-1939 by physicists Hans Bethe and Carl Friedrich von Weizsäcker. The name CN or CNO cycle are derived from the elements involved in the reaction of carbon (C ), nitrogen ( N ) and oxygen (O). While the proton-proton reaction plays an important role in stars with sizes up to the mass of the Sun, theoretical models suggest that the Bethe - Weizsäcker cycle probably is the dominant energy source in heavier stars. The sun itself produces only 1.6 % of their energy through the Bethe- Weizsäcker cycle.

The Bethe- Weizsäcker - cycle only runs at temperatures above 14 million Kelvin, and is from 30 million Kelvin predominant. The turnover rate is in direct proportion to the existing amount of C12C.

Since, according to current opinion of the Big Bang no carbon could occur, it was the stars of the first generation ( Population III ) impossible to produce energy in this way. In the late stages of stellar evolution, however, carbon is formed from the stars by the three- alpha process (see nucleosynthesis ), the then firstly is available as a catalyst is discharged to another by supernovae to the interstellar medium from which new stars form.

Therefore, stars of later generations already contained in its infancy carbon ( see also metallicity ).

In the Bethe- Weizsäcker cycle take place mainly mergers of hydrogen nuclei 1H ( protons) with the heavier nuclei 12C, 13C, 14N and 15N, hence the name CN- cycle. The fusion of partially energy in the form of gamma rays is emitted γ. Two of the resulting intermediates, 13N and 15O, are unstable and decompose after a short time, each with emission of a positron e and an electron neutrino νe. The individual reaction steps are listed below. Column indicates the mean life time of the particles to respond to the left sides of equations according to:

Overall result of the cycle is the fusion of four hydrogen nuclei 1H to form a helium nucleus 4He2 ( α - particles ), whose mass is lower by about 1% than the mass of four protons ( mass defect ). The difference is this mc ² almost completely converted into energy according to the Einstein's equation E =. The energy balance is 25.03 MeV here. The carbon nucleus 12C serves only as a catalyst and is finally regenerated with the last reaction. The energy that carry the neutrinos in the form of their low mass and especially their kinetic energy is removed from the star, since they can escape through the stellar matter through almost unhindered.

A complete run of the cycle requires enormous periods of time - in the order of hundreds of millions of years with massive stars. The cycle runs at massive stars more rapidly than the proton-proton reaction ( few billion years ), so stars can release will use much more energy.

The energy generation rate is the Bethe- Weizsäcker cycle proportional to the 15th power of the temperature. Consequently causes an increase of the temperature by 5%, an increase of 108% in the release of energy.

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

See also: proton-proton reaction, 3α - process carbon burning, Neonbrennen, oxygen burning, silicon burning