Proton–proton chain reaction

The proton-proton reaction (pp reaction, proton-proton chain) is one of two fusion reactions of the so-called hydrogen burning, through which convert hydrogen into helium star. The other reaction is the Bethe- Weizsäcker - cycle (CNO cycle). For stars with sizes up to the mass of the Sun, the proton -proton reaction plays an important role in energy conversion. The highly exothermic nature of the fusion stems from the fact that the final helium is lower by 0.635 % mass than the hydrogen particles received in the reaction ( mass defect ). The difference is converted into energy according to the Einstein's equation E = mc2.

The proton-proton reaction has the lowest temperature conditions occurring in stars of all fusion reactions (though in brown dwarfs also run below this limit from fusion reactions, but they do not count to the stars ). You can proceed in stars with a core temperature of more than 3 million Kelvin. At these temperatures all the nuclei concerned are completely ionized, i.e., without the electron shell.

The fusion rate is proportional to the proton-proton reaction to the 6th power of the temperature. Consequently causes an increase of the temperature by 5%, an increase of the release of energy of 34%.

• 3.1 proton-electron -proton reaction
• 3.2 helium -proton reaction

Start reactions

First, two hydrogen nuclei fuse 1H ( protons ) to a deuterium nucleus 2H, where the conversion of a proton into a neutron, a positron e and an electron neutrino νe are free.

The reaction rate is very low and therefore the rate-determining for the overall reaction. This is because the electrostatic repulsion of the positively charged protons keeps mostly on distance, there is no state for the bound diproton and the emergence of the neutron is defined as the process of the weak interaction is possible only at very small intervals. Very rare, even after the Maxwell - Boltzmann distribution, particularly high-energy collisions are not sufficient according to classical theory. Only by the quantum mechanical tunneling effect, the protons come yet close enough, but with very low probability: In the sun it takes an average of 1.4 × 1010 years, until a certain proton reacts with another, which is why the sun has a great life.

From the relatively low energy release of the reaction, the neutrino carries an average of 0.26 MeV it. Since these light particles penetrate the stellar matter almost unhindered, this amount of energy for the stellar physics is lost.

The resulting positron annihilates instantly with an electron e -, i.e., they react with each other and are totally converted into the energy. The mass of both partners is γ in the form of two gamma quanta energy.

The resulting deuterium can then react with another proton, wherein the light helium isotope 3He arises:

This process is not dependent on the weak interaction, and the bond energy is large. Therefore, the reaction rate is much higher: In the sun the deuterium resulting from the initial reaction only live about 1.4 seconds. The existing in the star formation deuterium can already react in much smaller celestial bodies, from a size of about 12 Jupiter masses. This marks the lower limit for a brown dwarf.

Main secondary reactions

There are now essentially three different reaction chains in which finally the (in nature overwhelming ) helium isotope 4He is produced. Insert at different temperatures. In the sun, the reactions described below occur with varying frequency on:

• Proton-proton reaction I: 91 %
• Proton-proton reaction II: 9%
• Proton -proton reaction III: 0.1%

Proton-proton reaction I

After an average of 106 years to merge two helium nuclei 3He to 4He ( α - particles ), which are two free protons. They are available for further reaction steps.

The complete reaction chain up here in the reactions listed in the start reaction are run through twice each in order to create the necessary 3He particles for the last merger, is a net energy - ie minus the neutrino energy - from

Free ( ≈ 4.20 · 10-12 J). The proton-proton reaction I is predominant at temperatures of 10-14 million Kelvin. Below this temperature, very little 4He is produced.

Proton-proton reaction II

In the proton-proton reaction II, a previously produced core serves 4He helium as a catalyst to produce a further of 3He.

Proton -proton reaction II runs mainly from at temperatures of 14 to 23 million degrees Kelvin.

90% of the neutrinos, which are generated by the second reaction, have an energy of 0.861 MeV, 0.383 MeV, while it is in the remaining 10%, depending on whether the resulting lithium 7Li is in the ground state or in an excited state.

The third reaction step, without the first two reactions occur with lithium, which mitbekam the star at its birth (lithium burning). This increases the concentration of lithium into stars.

Proton-proton reaction III

Again, a helium nucleus 4He acts as a catalyst.

The proton-proton reaction III is predominant at temperatures above 23 million Kelvin.

Although this reaction is not the main energy source of the sun, the temperature is not high enough for it, but it plays in the explanation of the solar neutrino problem, an important role because it produces neutrinos with the highest energies of up to 14.06 MeV, the so-called 8B neutrinos. Such neutrinos can be more easily detected in terrestrial neutrino detectors than the low-energy.

Further reactions

In addition to the three above-mentioned reactions, there are two rare running.

Proton-electron -proton reaction

In the proton-electron -proton reaction, short pep reaction, merge two protons and one electron to a deuterium nucleus.

The reaction occurs so rarely as to - in the sun at a ratio of 1:400 with respect to the proton-proton reaction I - since it must meet three particles almost simultaneously. The energy of the neutrinos produced is, however, considerably higher at 1.44 MeV.

Helium -proton reaction

Even more rarely, the helium -proton reaction occurs (short Hep reaction), the direct fusion of helium 3He with a proton to 4He.

Ash

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