Stellar nucleosynthesis

Stars begin with gas dominated by hydrogen and helium. In their hot, dense interiors, nuclear fusion converts lighter nuclei into heavier ones and releases energy. Hydrogen fusion makes helium; in stars that reach the necessary later stages, helium fusion can make carbon and oxygen. The sequence and products depend strongly on stellar mass and evolutionary stage. Massive stars can proceed through successive burning stages and build nuclei up to iron-group elements in their cores. Fusion of iron does not provide the energy that supports a star in the way earlier fusion stages do, so an iron-rich core in a massive star is associated with the approach to core collapse rather than a further energy-producing fusion stage.

Fusion is not the only route to new nuclei. In neutron-capture processes, atomic nuclei absorb neutrons and may later undergo radioactive beta decay, changing their proton number and therefore their element. The slow neutron-capture process (s-process) proceeds when neutron captures are generally slower than the relevant beta decays; it operates in particular evolved stars. In the rapid neutron-capture process (r-process), captures occur rapidly compared with beta decay, producing very neutron-rich nuclei that subsequently decay toward stable isotopes. Evidence and models associate r-process production with some explosive environments, including neutron-star mergers; the relative contribution of different sites remains an active research subject. These processes and other pathways help explain why the periodic table cannot be attributed to ordinary stellar-core fusion alone.

Newly made nuclei affect the wider universe only when material is returned to surrounding space. Evolved low- and intermediate-mass stars can shed enriched outer layers, while massive stars eject matter in winds and, at the end of their lives, in supernova explosions. Compact-object mergers can also eject neutron-rich material. This matter mixes with interstellar gas, from which later stars and planetary systems form. As a result, the chemical composition of galaxies changes over time; astronomers use the term metallicity for the abundance of elements heavier than helium in astronomical material.

Different elements and isotopes record different astrophysical histories. Their observed abundances, stellar spectra, meteorites and transient events provide complementary evidence used to test models of nucleosynthesis. No single star type or explosion accounts for every element: some are built mainly through fusion, others through neutron capture or additional proton-rich pathways, and many have contributions from more than one source. Accordingly, element-origin diagrams are summaries of scientific estimates, not simple one-to-one assignments.