AGN accretion disk
/ uh-KREE-shun /
You cannot drop a stone straight down a well if the stone is already circling the rim — it keeps orbiting. Gas trying to fall into a black hole has the same problem: it almost always arrives with some sideways motion, so instead of plunging straight in it settles into a flat, spinning disk and slowly spirals inward. That swirling pancake of infalling gas is the accretion disk, and it is the part of an AGN that actually shines.
As gas spirals in, friction and turbulence make neighboring rings rub against each other and heat up tremendously — the inner edge can reach hundreds of thousands of degrees, hot enough to glow in ultraviolet and soft X-rays. The disk is extraordinarily efficient: falling matter can convert several to tens of percent of its rest-mass energy (E = mc^2) into light, dwarfing the roughly 0.7 percent that nuclear fusion squeezes out of hydrogen in stars. To outshine a galaxy, a supermassive black hole need only swallow about one Sun's worth of gas per year. The disk is small — for a billion-solar-mass black hole, light-days to light-weeks across — which is why AGN can flicker so quickly.
The accretion disk is the AGN's furnace and the reference point for everything else: the corona hovers just above it, the broad-line region orbits farther out, and jets (when present) launch from its innermost edge. Its inner boundary lies near the innermost stable circular orbit, set by the black hole's mass and spin, so reading the disk's hottest light lets astronomers probe the warped spacetime nearest the hole. Real disks are messy and not perfectly understood — exactly how angular momentum is shed so gas can fall is still an active research question — but the basic 'spiral in, heat up, shine' picture is solid.
A quasar's blue-white glare comes mainly from its accretion disk's hot inner regions. The most luminous quasars shine at 10^40 watts or more — yet by counting how fast their light changes, astronomers infer the emitting disk is only light-days across, smaller than the distance from the Sun to its nearest neighbor.
A glowing whirlpool of infalling gas — the AGN's actual light source.
The disk does not glow because the black hole 'lights up'; the black hole itself emits nothing. The light is gravitational energy released by gas heating as it spirals down, the same physics as the accretion disks around stellar-mass black holes, just vastly bigger.