stellar nucleosynthesis

The universe began with almost nothing but hydrogen and helium. Nearly every heavier atom, the carbon in your cells, the oxygen you breathe, the iron in your blood, the calcium in your bones, was forged inside stars and scattered when they died. Stellar nucleosynthesis is the network of nuclear reactions by which stars build the chemical elements. We are, quite literally, made of stardust.

The story climbs the binding-energy curve step by step. Hydrogen burns to helium by the proton-proton chain and the CNO cycle. When the core runs out of hydrogen and contracts and heats, helium fuses via the triple-alpha process (three helium-4 nuclei -> carbon-12), then carbon, neon, oxygen, and silicon burn in successive shells of ever more massive stars, building up to the iron peak. There the road ends for energy-releasing fusion, because iron-group nuclei sit at the maximum binding energy per nucleon: fusing them further absorbs energy rather than releasing it. Elements heavier than iron are therefore built not by ordinary fusion but by neutron capture, the slow s-process in evolved stars and the rapid r-process in explosive environments, followed by beta decays that raise the proton number.

The heaviest elements, gold, platinum, uranium, are minted in the most violent events: core-collapse supernovae and, as gravitational-wave observations confirmed, neutron-star mergers, where intense neutron fluxes drive the r-process. This framework, laid out in the classic B2FH paper by Burbidge, Burbidge, Fowler and Hoyle, ties the abundance of every element to nuclear physics. Note the division of labour: Big Bang nucleosynthesis made only the lightest nuclei (hydrogen, helium, a trace of lithium) in the first minutes, while everything heavier is the ongoing work of stars.