nucleosynthesis

Nucleosynthesis is the formation of atomic nuclei from pre-existing nucleons in cosmic settings. It encompasses fusion, neutron capture, and other nuclear reactions occurring in stars, stellar remnants, and the early universe. The term contrasts with nuclear decay, which reduces nuclei to lighter species.

Big Bang nucleosynthesis occurred within minutes after the Big Bang, producing most of the universe's light elements: hydrogen, helium-4, helium-3, deuterium, and trace lithium-7. Predicted abundances depend on the baryon density and agree with values inferred from the cosmic microwave background, giving a key test of cosmology and particle physics.

Stellar nucleosynthesis powers stars and builds heavier elements. Hydrogen burning converts H to He in main-sequence stars; subsequent burning cycles in more massive stars synthesize elements up to iron via helium, carbon, neon, oxygen, and silicon burning. Iron-peak elements are formed when energy production by fusion ceases, and later supernovae and related explosions eject many of these elements into the interstellar medium.

Beyond fusion, nucleosynthesis also proceeds via neutron capture. The slow s-process builds heavier nuclei gradually in asymptotic giant branch stars, while the rapid r-process occurs in explosive environments such as neutron star mergers or core-collapse supernovae, creating many neutron-rich isotopes. The p-process forms rare proton-rich isotopes, contributing to the observed isotopic diversity.

Additional production mechanisms include cosmic ray spallation, which creates light elements like lithium, beryllium, and boron. The observed abundance patterns across galaxies reflect galactic chemical evolution and the cumulative history of star formation, stellar deaths, and element recycling in the cosmos.