Proton-proton fusion - Definition and Overview

The proton-proton reaction--the first step in the proton-proton chain. Two protons fuse to form a deuterium nucleus. The reaction results in the emission of a positron and a neutrino.

In the second step of the chain, a single proton fuses with a deuterium nucleus, resulting in 3He and a gamma ray

The proton-proton chain reaction is one of two fusion reactions by which stars convert hydrogen to helium, the other being the CNO cycle. The proton-proton chain is more important in stars the size of the Sun or less.

To overcome the electromagnetic repulsion between two hydrogen nuclei requires a large amount of energy, and this reaction takes an average of 10 billion years to complete. Because of the slowness of this reaction the Sun is still shining; if it were faster, the Sun would have exhausted its hydrogen long ago.

The first step involves the fusion of two hydrogen nuclei ¹H (protons) into deuterium ²H, releasing a positron as one proton changes into a neutron, and a neutrino.

¹H + ¹H → ²H + e⁺ + ν_e + 0.42 MeV

The positron immediately annihilates with one of the hydrogen's electrons, and their mass energy is carried off by two gamma ray photons.

e⁺ + e^- → 2γ + 1.02 MeV

After this the deuterium produced in the first stage can fuse with another hydrogen to produce a light isotope of helium, ³He:

²H + ¹H → ³He + γ + 5.49 MeV

Finally, after millions of years, two of the helium nuclei ³He produced can fuse together to make the common helium isotope ⁴He, releasing two hydrogen nuclei to start the reaction again through three different paths called PP1, PP2 and PP3:

PP1:

³He +³He → ⁴He + ¹H + ¹H + 12.86 MeV

The complete PP1 chain reaction releases a net energy of 26.7 MeV. The PP1 chain is dominant in temperatures of 10 to 14 megakelvins (MK). Below 10 MK, the PP chain does not produce much ⁴He.

PP2:

The PP2 chain is dominant in temperatures of 14 to 23 MK. PP3:

The PP3 chain is dominant if the temperatures exceeds 23 MK.

The PP3 chain is not a major source of energy in the Sun (as the Sun's core temperature is not high enough), but was very important in the solar neutrino problem because it generates the highest energy neutrinos.

In general, proton-proton fusion can occur only if the temperature (i.e., kinetic energy) of the protons is high enough that they can overcome the mutual Coulomb force repulsion. The theory that proton-proton reactions were the basic principle by which the Sun and other stars burn was advocated by Arthur Eddington in the 1920s. At the time, the temperature of the Sun was considered too low to overcome the Coulomb-force barrier. After the development of quantum mechanics, it was discovered that the tunneling of the wave functions of the protons through the repulsive barrier allowed for fusion at a lower temperature than the classical prediction.

See also:

• Triple-alpha process
• CNO cycle