The proton-proton chain reaction is one of the primary nuclear fusion processes that occurs in stars, particularly in the cores of low to medium-mass stars like our Sun. In this process, hydrogen nuclei (protons) combine to form helium nuclei through a series of fusion reactions. The proton-proton chain consists of three main branches, often referred to as the PP I, PP II, and PP III branches, each with distinct steps. The overall reaction is as follows:
4 protons (hydrogen nuclei) ⟶ 1 helium nucleus + energy.
Throughout the proton-proton chain, various subatomic particles called bosons and fermions are involved. The specific reactions and particles involved depend on the branch of the chain.
In the first branch (PP I), the reactions occur as follows:
Two protons (hydrogen nuclei) fuse to form a deuterium nucleus (one proton and one neutron) along with a positron (antielectron) and a neutrino: p + p ⟶ D + e^+ + νe.
The deuterium nucleus then fuses with another proton to form a helium-3 nucleus (two protons and one neutron): D + p ⟶ He-3 + γ (gamma-ray photon).
Two helium-3 nuclei fuse to form helium-4 (two protons and two neutrons) along with two protons: He-3 + He-3 ⟶ He-4 + 2p.
The second and third branches (PP II and PP III) involve different reactions, but they eventually converge back to the formation of helium-4 (He-4) and release energy.
Overall, the proton-proton chain reactions in stars, including the Sun, are responsible for the conversion of hydrogen into helium, releasing a tremendous amount of energy in the process. This energy is what powers the star, providing heat and light. The specific details of the proton-proton chain and the energy release have been studied through astrophysical observations, laboratory experiments, and theoretical models, contributing to our understanding of stellar evolution and the energy generation processes in stars.