Proton-proton chain

The proton-proton chain reaction (also known as the PP chain) 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.

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. 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. It is because the slowness of this reaction that the Sun is still shining; if it where faster, the Sun would have exhausted its hydrogen long ago.

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. PP1 chain is dominant in temperatures of 10-14 million Kelvin. Below 10 million Kelvin, the PP chain does not produce much ⁴He.

PP2:

	3He + 4He	→	7Be + γ
	7Be + e-	→	7Li + νe
	7Li + 1H	→	4He + 4He

PP2 chain is dominant in temperatures of 14-23 million Kelvin.

PP3:

	3He + 4He	→	7Be + γ
	7Be + 1H	→	8B + γ
	8B	→	8Be + e+ + νe
	8Be	↔	4He + 4He

PP3 chain is dominant if the temperatures exceeds 23 million Kelvin.

See also:

• Triple-alpha process