This illustration shows a swirling disk of accreting gas orbiting a black hole, with the bulk of the X-rays pouring out of the inner, white-shaded region of the disk. One remarkable prediction of Einstein's relativity theory is the existence of a smallest radius for the disk, inside of which the gas suddenly plunges into the hole with no time to radiate away its energy.
For the non-spinning black hole shown at left, this inner radius is large, which leaves a big dark hole cut out of the center of the hot disk of gas. For the fast-spinning black hole shown at right, the gas can orbit very near the event horizon, and thus only a small portion of the inner disk is missing. Therefore, the radius of the hole is a direct measure of the spin.