The first generation of stars — Population III — was extraordinarily massive: bodies of a hundred, even three hundred solar masses. Such size meant tremendous radiative power, but also brief lives. A Pop III star lived only a few million years from birth to destruction, ending in supernova.

What remained depended on mass. Lighter stars leave behind a neutron star; but those above a certain threshold collapse so completely that no known force can halt the contraction. A black hole forms. Because most Pop III stars crossed that threshold, the very first black holes in the universe began appearing around 13.5 billion years ago, alongside the death of the first stars.

Whether these early stellar-mass black holes were the seeds of today's supermassive ones is an open question. Some models say yes: a stellar remnant can grow into millions of solar masses across billions of years through accretion and mergers. But the unexpectedly luminous objects spotted in the very early universe by JWST suggest that these seeds grew faster than predicted — one of the unresolved puzzles of modern cosmology.