Stellar nucleosynthesis

Stellar nucleosynthesis begins with hydrogen fusion, the primary energy source for main-sequence stars like the Sun. In the proton–proton chain and the CNO cycle, hydrogen nuclei fuse into helium, releasing energy that balances gravitational collapse. When hydrogen is exhausted, helium burning produces carbon and oxygen via the triple-alpha process. In more massive stars, successive stages—carbon, neon, oxygen, and silicon burning—create elements up to iron. These stages occur at increasingly higher temperatures and densities, with each stage lasting shorter than the previous one.

Elements heavier than iron are not produced by fusion, as iron has the highest binding energy per nucleon. Instead, they are created by neutron capture processes: the slow (s-process) and rapid (r-process). The s-process occurs in asymptotic giant branch (AGB) stars, where neutrons are produced by reactions like ¹³C(α,n)¹⁶O, and builds elements up to bismuth over timescales of thousands of years.² The r-process, responsible for about half of the heavy elements, happens in explosive environments such as supernovae and neutron star mergers, where neutron fluxes are so high that nuclei capture many neutrons before decaying.

Supernovae are the most dramatic sites of nucleosynthesis. Core-collapse supernovae, which mark the death of massive stars, synthesize elements from oxygen to rubidium through explosive burning and the r-process. Type Ia supernovae, which occur in binary systems with white dwarfs, produce large amounts of iron-peak elements, particularly ⁵⁶Ni, which decays to ⁵⁶Fe. These events eject newly formed elements into the interstellar medium, enriching the gas from which future stars and planets form.³ Neutron star mergers, first confirmed in 2017, are now recognized as a major r-process site, producing elements like gold and platinum.

Beyond the main processes, stellar nucleosynthesis includes several subtle mechanisms. The νp-process, which operates in supernova ejecta, produces proton-rich isotopes of elements like molybdenum and ruthenium. The p-process, though less understood, creates rare proton-rich isotopes via photodisintegration. In low-metallicity stars, the s-process can operate differently due to the lack of iron seeds, leading to enhanced production of elements like barium. Additionally, the first stars (Population III) may have produced elements via the CNO cycle even without initial carbon, through the hot CNO cycle in massive stars. These processes contribute to the chemical evolution of galaxies, leaving distinct abundance patterns that astronomers use to trace stellar populations.⁴

s-process

Slow neutron capture process occurring in AGB stars, building heavy elements over long timescales.

r-process

Rapid neutron capture process occurring in explosive events, producing about half of heavy elements.

Triple-alpha process

Fusion of three helium nuclei into carbon, occurring in helium-burning stars.

CNO cycle

Hydrogen fusion cycle using carbon, nitrogen, and oxygen as catalysts, dominant in stars more massive than the Sun.