The proton-proton (PP) chain reaction is one of the primary nuclear fusion processes that occurs in the cores of stars, including our Sun. Through a series of nuclear reactions, the PP chain converts hydrogen nuclei (protons) into helium nuclei, releasing a tremendous amount of energy in the process. The PP chain has several stages, each contributing to the production of helium:
Stage 1: Two protons (hydrogen nuclei) combine to form a deuterium nucleus (one proton and one neutron) through a weak nuclear force interaction. This step releases a positron (a positively charged electron) and a neutrino as byproducts.
Stage 2: A deuterium nucleus and a proton fuse to form a helium-3 nucleus (two protons and one neutron). This reaction also generates a photon (a gamma ray) as a byproduct.
Stage 3: Two helium-3 nuclei combine, resulting in the formation of a helium-4 nucleus (two protons and two neutrons). This process releases two protons, which can then go through further reactions to continue the chain, along with two excess protons that are not involved in the current chain.
The overall reaction can be summarized as follows:
4 protons (hydrogen nuclei) -> helium-4 nucleus + 2 positrons + 2 neutrinos + energy
The released energy is in the form of gamma rays (high-energy photons) resulting from the nuclear reactions. This energy is crucial for maintaining the high temperatures and pressures needed to sustain the star's stability and prevent gravitational collapse.
The proton-proton chain reaction is the dominant fusion process in stars like the Sun, where the core temperature is not high enough to support more energetically demanding fusion reactions like the CNO cycle (carbon-nitrogen-oxygen cycle).