Once the early radiation freed itself, the universe slid into darkness: no stars yet shone, and the sky was nothing but a slowly cooling gas. This "cosmic dark age" lasted some two hundred million years. Gravity worked patiently on tiny density variations; giant clouds of hydrogen and helium collapsed until their cores crossed the pressure threshold needed to ignite fusion.

These earliest stars — called Population III, or Pop III — were unlike anything in today's sky. With no heavier elements yet produced, they were nearly pure hydrogen and helium, could reach tens or even hundreds of solar masses, and tore themselves apart within a few million years in enormous supernovae. Those explosions seeded the universe with carbon, oxygen, iron and the other heavy elements; later stars, planets, and ultimately life inherited their chemistry from them.

Pop III stars have never been observed directly. We know of them because cosmological models require them — and because instruments like the James Webb Space Telescope are now catching the light of the earliest galaxies built from their remains.