Ergosphere

The ergosphere is a region located outside a rotating black hole. Its name is derived from the Greek word ergon, which means “work”. It received this name because it is theoretically possible to extract energy and mass from the black hole in this region. The ergosphere has an oblate spheroidal shape and is situated so that at the poles of a rotating black hole it touches the event horizon and at the equator it stretches out to a distance equal to the radius of the event horizon. The equatorial (maximum) radius of an ergosphere corresponds to the Schwarzschild radius of a non-rotating black hole; the polar (minimum) radius can be as little as half the Schwarzschild radius if the black hole is rotating maximally.