Nucleosynthesis

Nucleosynthesis is the process by which the chemical elements of the universe are created through nuclear reactions in stars, supernovae, and other astrophysical environments. The light elements — hydrogen, helium, and trace lithium — were formed in the first minutes after the Big Bang through Big Bang nucleosynthesis. All heavier elements are produced by stellar nucleosynthesis, in which stars fuse lighter nuclei into heavier ones through successive stages of nuclear burning.

The heaviest elements — those beyond iron — are synthesized primarily through the rapid neutron capture process (r-process) in supernovae and neutron star mergers. This process requires extreme neutron fluxes that only exist in the most violent astrophysical events. The abundance of elements on Earth and in our bodies is therefore a direct record of the supernova history of the Milky Way.

Nucleosynthesis is often presented as a chain of nuclear reactions, but this misses the systems aspect. The production of heavy elements is not merely a sequence of nuclear reactions but a consequence of the thermodynamic disequilibrium that stars create and maintain. The star is a machine for transforming hydrogen into heavier elements, and the supernova is the mechanism that delivers the products.