A quasar is a supermassive black hole feeding on gas at the center of a distant galaxy.
Quasar is short for quasi-stellar radio source, because astronomers first discovered quasars in 1963 as objects that looked like stars but emitted radio waves. Now, the term is a catch-all for all feeding, and therefore luminous supermassive black holes, also often called active galactic nuclei.
It’s a bit of a contradiction to call a black hole luminous; black holes themselves are, of course, black. In fact, almost every large galaxy hosts a black hole with the mass of millions to billions of Suns, and many of these black holes lurk in the dark. Our Milky Way’s behemoth weighs in at 4.3 million solar masses, but its starvation diet mutes all but faint flashes and flickers. We know it’s there, though, from the orbits of stars around it. Other dormant black holes occasionally shred an infalling star, making their presence known by the flare of radiation that ensues.
But quasars are a different breed of black hole. They reside in galaxies with plentiful gas supplies, perhaps supplied by a recent galaxy-galaxy collision, and they gorge on the inflowing material. The gas spirals around as it falls in, heating up in the process and emitting radiation across the electromagnetic spectrum.
Supermassive black holes in nearby galaxies typically do not have that much gas available to them, so quasars are typically found in distant galaxies. The nearest quasar is Markarian 231, which lies about 600 million light-years from Earth.
What Does a Quasar Look Like?
A quasar is not only the feeding black hole itself, but the light-producing structures that surround it. Visible and ultraviolet light come from the glowing disk of infalling material, while even hotter gas above the disk shines at X-ray energies. Jets shooting out along the black hole’s poles emit everything from radio waves to X-rays. Farther out from the black hole, the prolific dust and gas glow at infrared wavelengths.
The size of a quasar accretion disk, which scales with the mass of its black hole, is typically a few light-days across. That dwarfs in comparison to its host galaxy; the Milky Way for comparison is roughly 100,000 light-years across. Yet quasars often outshine their hosts.
Despite their brilliance, quasars are so small and distant that even the most powerful telescope cannot resolve all the structures within a quasar. Astronomers have to ferret out the details using other techniques, such as analyzing spectroscopy (spreading out the light by wavelength) or light curves (spreading out the light by its arrival time).
While the details are still up for debate, we can use current knowledge to paint a general picture of a quasar. Just remember that this picture might change over time as we learn more!