Hydrogen burning

Scheme of the proton-proton reaction

With hydrogen burning is fusion of protons (d. E. Of atomic nuclei of the most common isotope of ¹ H of the hydrogen ) to helium in the interior of stars (in the case one or, Nova , on the surface of a white dwarf ) refers, in other words, the stellar hydrogen fusion . This reaction is the main source of energy in normal stars for most of their life cycle. All stars in the main sequence get their energy from burning hydrogen. Despite the name, it is not a combustion in the sense of the chemical redox reaction , it releases significantly less energy.

Hydrogen burning can be summarized as follows:

${\displaystyle 4{}_{1}^{1}\mathrm {H} \rightarrow {}_{2}^{4}\mathrm {He} +2e^{+}+2\nu _{e}+2\gamma }$,

When four protons are fused, two positrons , two electron neutrinos and two gamma quanta are created in addition to the helium nucleus . Due to the mass defect that occurs , an energy of 26.731  MeV is released. The direct fusion of four protons is too improbable to explain the luminosity of the stars; instead, hydrogen burning takes place primarily in two different reaction chains:

• the relatively direct proton-proton reaction
• the Bethe-Weizsäcker cycle (CNO cycle) using heavy elements ( carbon , nitrogen , oxygen )

The electron neutrinos can leave the star practically unhindered, so the energy available to the star depends on the reaction chain

Relative energy production for the proton-proton (PP), CNO and triple-α fusion processes as a function of temperature. At temperatures like the core of the sun, the PP process is dominant.
Attention: temperature scale is incorrect!

The rate of energy production is proportional to the fourth power of temperature in the proton-proton reaction, and to the 18th power in the Bethe-Weizsäcker cycle . An increase in temperature by 5% therefore causes an increase of 22% or 141% in the energy release. With helium burning (27th power) and carbon burning ( 30th power) these values ​​are even higher.

During the main sequence phase, hydrogen burning takes place in the core of the star at temperatures between 5 and 55 MK. For the sun, this means that at a core temperature of 15.6 MK, around 564 million tons of hydrogen are “fused” into 560 million tons of helium every second, so the mass defect is 4 million tons. After leaving the main row, the hydrogen burning takes place in a shell around the core. Temperatures between 45 and 100 MK are reached.

The mass defect in the fusion of hydrogen to helium is the largest of all fusion reactions and therefore the most productive in terms of energy; the next stage of stellar fusion reactions, the burning of helium , only releases about a tenth of this per carbon nucleus produced.

literature

• Bogdan Povh et al .: Particles and Cores. 4th edition. Springer Verlag 1997, ISBN 3-540-61737-X , pp. 317-318.

Individual evidence

1. ^ Bradley W. Carroll, Dale A. Ostlie: An introduction to modern astrophysics . 2nd ed., Pearson new International ed. Pearson, Harlow 2014, ISBN 978-1-292-02293-2 , pp. 399 .
2. ↑ ^{a b c d e} Christian Iliadis: Nuclear Physics of Stars . 2nd Edition. Wiley-VCH, Weinheim 2015, ISBN 978-3-527-33648-7 , pp. 353 (English).
3. ^ John N. Bahcall , M. C. Gonzalez-Garcia, Carlos Peña-Garay: Does the Sun Shine by pp or CNO Fusion Reactions? In: Physical Review Letters. 90, 2003, doi : 10.1103 / PhysRevLett.90.131301 .
4. ^ Christian Iliadis: Nuclear Physics of Stars . 2nd Edition. Wiley-VCH, Weinheim 2015, ISBN 978-3-527-33648-7 , pp. 364 (English).
5. ↑ 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 .