Quasar

In the late 1950s radio astronomers found sources in the sky that looked, through optical telescopes, like ordinary faint stars. But their light made no sense. In 1963 Maarten Schmidt realised why: the spectrum of one of them, 3C 273, was shifted towards the red by about 16 percent. It was not a star in our galaxy at all, but something billions of light-years away, and to look as bright as it did from there it had to be shining with the light of trillions of suns. These “quasi-stellar objects” became known as quasars.

What powers them

A quasar is the centre of a galaxy where a supermassive black hole, weighing millions to tens of billions of times the Sun, is swallowing gas. The gas spirals in, forms a disk, and heats up so much that it outshines every star in the galaxy around it. 3C 273 alone gives off about four trillion times the Sun’s light, roughly a hundred times as much as a giant galaxy like the Milky Way. Its brightness changes within a year, so the region producing that light is less than a light-year across.

Lights from the early universe

More than 900 000 quasars are known. Almost all of them are very far away, which means we see them as they were long ago: quasar activity peaked about ten billion years ago, when galaxies were young and full of gas. The nearest, Markarian 231, is still about 600 million light-years away.

The brightest of all

In February 2024 Christian Wolf and colleagues announced that J0529−4351, a quasar about 12 billion light-years away, is the most luminous object ever observed, shining with roughly 500 trillion times the Sun’s light. Its black hole appeared to be devouring about one Sun’s worth of gas every day. It had been hiding in plain sight: a 2022 survey had classified it as an ordinary star in our own galaxy. A 2025 estimate put its black hole at about a billion solar masses, much less than the 17 billion first reported.