Why We Are Literally Made of Stardust

In the first few minutes after the Big Bang, the universe was hot and dense enough for nuclear fusion, but it cooled too fast for much to happen. The result was roughly 75% hydrogen and 25% helium by mass, with only trace amounts of lithium. No carbon, no oxygen, no nitrogen, no iron — none of the elements life is built from.

A star spends its life fusing hydrogen into helium in its core, releasing the energy that makes it shine. When the core hydrogen runs low, a massive star starts fusing helium into carbon, then carbon into oxygen, neon, silicon, and onward — building heavier elements in successive shells, up to iron. Iron is where fusion stops paying its own energy bill, so the process halts there.

A massive star ends in a supernova: an explosion that both forges elements heavier than iron in a few violent seconds and blasts the star's entire accumulated inventory of elements out into space. Lower-mass stars like the Sun end more gently, puffing their outer layers off as a planetary nebula. Either way, enriched gas returns to the galaxy.

That enriched gas mixes into clouds that collapse to form new stars — and planets, and eventually chemists who can work all this out.

The specific origins of the atoms in a human body are now well understood.

The atoms doing the reading right now are older than the Solar System and were assembled in places that no longer exist. Astrology and cosmology usually get treated as opposites — one poetry, one physics — but this is a case where the literal science is at least as awe-inspiring as any horoscope.