After the Big Bang, the universe contained only three elements: hydrogen, helium, and traces of lithium. None of the matter of today's rocks, planets, the iron in our blood, the calcium in our bones existed yet. All those elements were forged inside stars, in furnaces of pressure and millions-of-degrees heat.

The first stars — called Population III in the astrophysics literature — were very different from today's. Gas clouds with no metals (anything heavier than helium) could not cool efficiently, so the first stars formed extremely massive, often hundreds to thousands of solar masses. Such giants do not live long. In their cores they fused hydrogen into helium, helium into carbon, oxygen, neon, magnesium, and finally silicon and iron. Iron is the most stable nucleus, so fusion stops there; the core can no longer support itself and collapses under its own gravity.

The collapse ends in a supernova explosion. In the extreme temperatures of the blast, elements heavier than iron — copper, zinc, gold, uranium — are synthesised in seconds, and the star's layers are blown into space. The first wave of Pop III supernovae likely began 200–400 million years after the Big Bang. Their ejected heavy elements enriched the interstellar gas; from this enriched gas the second generation (Pop II) and all subsequent stars — including the Sun — formed.

Every piece of matter you touch today, the oxygen in your water, the minerals passing you in the street, was once inside a star's core. Most iron atoms now appear to come from neutron-star mergers, but life's structural elements like carbon and oxygen are largely the work of Pop III and later supernovae. This is the astrophysics behind Carl Sagan's line: "We are made of star stuff."