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.