Proton–proton chain

The proton–proton chain is a sequence of nuclear reactions that fuses hydrogen into helium. It supplies most of the energy produced by the Sun and other relatively cool stars.

Proton–proton reaction: A weak-interaction fusion reaction in which two protons form deuterium, a positron, and an electron neutrino. This slow first step initiates the chain and helps set the Sun’s fusion rate.
Nuclear fusion: A nuclear reaction in which light atomic nuclei combine to form a heavier nucleus. The proton–proton chain is one specific set of fusion reactions.
Solar neutrino problem: The historical discrepancy between predicted and detected solar neutrino fluxes, resolved by neutrino flavor change. Its resolution confirmed that neutrinos produced by the Sun change flavor before detection.
Triple-alpha process: A stellar fusion process that combines three helium-4 nuclei to form carbon-12. It consumes helium ash rather than hydrogen, so it belongs to a later stage of stellar evolution.
Deuterium: A stable hydrogen isotope whose nucleus contains one proton and one neutron. The chain’s first reaction creates deuterium, which can then capture another proton.
Coulomb barrier: The electrostatic energy barrier that positively charged nuclei must overcome to approach closely enough for fusion. Protons repel one another, so fusion requires access through this barrier.
Solar neutrino: A neutrino produced by nuclear reactions in the Sun’s interior. Solar neutrino energies identify contributions from particular proton–proton chain reactions.
Proton–proton chain branch I: The branch in which two helium-3 nuclei fuse to form helium-4 and two protons. This branch dominates hydrogen burning in the Sun.
Helium-3: A helium isotope with two protons and one neutron in its nucleus. Two helium-3 nuclei fuse in the main branch to produce helium-4 and return two protons.
Quantum tunneling: A quantum effect that allows a particle to pass through an energy barrier it could not cross classically. Tunneling lets solar protons fuse despite temperatures too low to overcome their repulsion classically.

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