The universe began with hydrogen and helium, plus a trace of lithium. Everything else was made in stars. Stellar nucleosynthesis is the process by which stars forge heavier elements through nuclear fusion. It is the reason carbon, oxygen, iron, and gold exist at all.

The process happens in stages. Main-sequence stars fuse hydrogen into helium. When the core hydrogen is exhausted, the star contracts and heats up, and helium fusion begins, producing carbon and oxygen. Massive stars continue fusing heavier elements in shells around the core. Each stage is shorter than the last. Iron is the end of the line. Fusing iron consumes energy rather than releasing it, so the core collapses.

The collapse triggers a supernova, and the explosion forges elements heavier than iron through rapid neutron capture. Those elements are scattered into space, where they mix with interstellar gas and become part of new stars and planets. The gold in a wedding ring was made in a supernova or a neutron star merger. The iron in your blood was made in a dying star.

Stellar nucleosynthesis connects the life cycle of stars to the composition of the universe. Without it, the universe would be hydrogen and helium, and nothing else.