Heavier elements beyond hydrogen and helium are primarily formed through nuclear processes in stars and during cosmic events. Elements up to iron are made by nuclear fusion inside stars, where lighter nuclei combine under extreme temperatures and pressures. For elements heavier than iron, two major neutron-capture processes play key roles: the slow neutron-capture process (s-process), which occurs in aging stars of 1 to 10 solar masses, and the rapid neutron-capture process (r-process), which requires intense neutron fluxes found in supernova explosions or collisions of neutron stars. The r-process is responsible for creating very heavy elements like gold, platinum, and uranium by rapid capture of neutrons onto seed nuclei, producing unstable nuclei that decay into heavy stable elements. Neutron star mergers, confirmed observationally by detecting gravitational waves and electromagnetic signals, are a major source of the heaviest elements in the universe. This cycle of element formation started with the Big Bang producing mainly hydrogen and helium, followed by stellar fusion and neutron capture processes enriching the universe with heavier elements that eventually form new stars, planets, and all matter.
