There is iron in your blood right now, perhaps four or five grams, enough to make a small nail. Every atom of that iron is older than the Earth, older than the Sun, and older than the Milky Way. But in the early universe, none of it existed. The cosmos contained only hydrogen and helium, drifting in vast, dark clouds across expanding space. This episode explores how the universe learned to make the heavy elements that eventually became part of you.

We trace the story back to the first generation of stars, known as Population III. Because the early universe lacked the heavier elements that help modern gas clouds cool and fragment, these ancient stars could not break apart as they collapsed. Instead, they formed as monstrous, incredibly hot objects that burned through their fuel in just a few million years. We examine what happened inside the ...