The name quasar (quasi-stellar object) was coined in the 1960s to describe star-like points in the sky at extraordinary distances. We now know quasars are supermassive black holes accreting hot gas; as the gas spirals in, friction makes it glow, and the disc radiates enormous amounts of radio, ultraviolet, and X-ray light. A typical quasar outshines the combined light of the hundreds of billions of stars in its host galaxy.

The striking puzzle is timing. Deep observations from ground-based and space telescopes show that black holes of billions of solar masses already existed within a billion years or less of the Big Bang. Early quasars known by names like ULAS J1120+0641, J0313–1806, and J1342+0928 push the limits of how black holes can grow so fast. Two main hypotheses dominate: (1) continuous mergers of large black-hole remnants of Pop III supernovae, and (2) direct collapse of huge gas clouds in the early universe straight into black holes, skipping the stellar stage.

Quasars are not just exotic spectacle. By heating and blowing away surrounding gas they regulate star formation in their host galaxies and reshape the intergalactic medium. Some models propose that a significant fraction of the ultraviolet light driving reionisation came from early quasars.

The light from quasars we observe today set out billions of years ago; to see one is to view a direct photograph of the early universe. The James Webb Space Telescope has begun measuring quasars more than 13 billion years old — opening this era as a laboratory for direct study.