A quasar is an active galactic nucleus: a supermassive black hole accreting matter so rapidly that its surrounding disk outshines all the stars in its host galaxy. The name comes from "quasi-stellar radio source".

Why are quasars so bright?

Accretion onto a black hole converts up to about 30% of infalling mass into radiation, compared with 0.7% for hydrogen fusion. Combined with the huge amounts of gas available, this makes quasars the most luminous persistent objects known.

How far away are quasars?

Most are billions of light-years away, and the most distant are seen as they were less than 700 million years after the Big Bang. Their extreme distance is why they look like faint stars despite their true brightness.

Is there a quasar in the Milky Way?

Not now. Sagittarius A* is dormant, accreting very little. It may have gone through quasar-like active phases in the past, and evidence of past outbursts is visible in the Fermi bubbles above and below the galactic plane.

What is a Quasar?

A quasar is a supermassive black hole in the act of feeding, surrounded by an accretion disk so hot and so luminous that it drowns out every star in its host galaxy. The brightest known examples radiate more than a hundred trillion times the Sun's output from a region not much larger than the Solar System.

The engine is gravity, not fusion. Gas spiralling toward the hole is compressed and sheared until friction heats it to hundreds of thousands of degrees, and accretion converts up to about 30% of the infalling mass into radiation — an order of magnitude more efficient than the nuclear fusion that powers stars. The disk radiates fiercely across the ultraviolet and X-ray bands, and magnetic fields anchored in the inner disk fling twin jets of plasma out along the poles at almost the speed of light.

They were discovered by accident. In the early 1960s astronomers found intense radio sources that looked like ordinary stars but had spectra nobody could read — hence "quasi-stellar object". In 1963 Maarten Schmidt realised the lines in 3C 273 were ordinary hydrogen, redshifted enormously. The object was billions of light-years away, which meant it was absurdly bright, which meant something entirely new was going on.

Because they are visible across nearly the whole observable universe, quasars are cosmological probes. The most distant known are seen as they were less than 700 million years after the Big Bang, already hosting billion-solar-mass black holes. The record for sheer output belongs to J0529-4351, identified in 2024, which swallows roughly a Sun's worth of matter every day. Quasar phases are brief: the object flares for perhaps 10 to 100 million years, then the fuel runs out and the galaxy goes quiet.