accretion disk
/ ak-KREE-shun /
When gas falls toward a dense object — a white dwarf, neutron star, or black hole — it almost never falls straight in. Like water swirling toward a drain, the gas has some sideways motion, so it spirals inward and flattens into a spinning disk. This is an accretion disk: a whirlpool of hot gas orbiting a compact object, slowly draining inward turn by turn. It is one of the most important structures in astrophysics, because it is where falling matter gives up its energy and lights up.
The magic is the heat. As gas in the disk rubs against itself through friction and turbulence, it heats up enormously — the inner edge of a disk around a black hole can reach millions of degrees and blaze in X-rays. Astonishingly, accretion is among the most efficient energy sources known: dropping matter onto a neutron star or black hole can convert up to about 10 to 40 percent of its mass-energy into radiation, dwarfing the roughly 0.7 percent that nuclear fusion extracts. A black hole eating gas is, gram for gram, a brighter engine than any star.
Accretion disks appear at every scale. Around a newborn star they birth planets; around a white dwarf they fuel novae; around a stellar-mass black hole they make an X-ray binary glow; around the supermassive black hole in a galaxy's heart they power a quasar bright enough to outshine a billion stars. The same physics — gas spiralling in, friction heating it, energy radiating away — runs all of them. A subtle point: the gas does not fall in because gravity simply 'sucks'; it must first shed angular momentum, which friction in the disk slowly does.
The black hole in the binary Cygnus X-1 cannot be seen, but the accretion disk it has torn off its giant companion blazes brightly in X-rays. That X-ray glow, from gas heated to millions of degrees as it spirals in, is how we know an invisible black hole is there at all.
We see the disk, not the black hole — its X-ray glow betrays the invisible.
Gas does not fall straight in because it carries angular momentum; the disk exists precisely so that friction can drain that spin away. Without that, matter would just orbit forever and never accrete.