ergosphere

thumb|right|300px|At the ergospheres (shown here in violet for the outer and red for the inner one), the temporal metric coefficient gtt becomes negative, i.e., acts like a purely spatial metric component. Consequently, timelike or lightlike worldlines within this region must co-rotate with the inner mass. Kerr–Newman metric#Alternative .28Kerr.E2.80.93Schild.29 formulation|Cartesian projection, equatorial perspective.

In astrophysics, the ergosphere is a region located just outside a rotating black hole, between its event horizon and a further external surface predicted by the Kerr metric which describes these objects. Its name was proposed by Remo Ruffini and John Archibald Wheeler during the Les Houches lectures in 1971 and is derived . It received this name because it is theoretically possible to extract energy and mass from this region. The ergosphere touches the event horizon at the poles of a rotating black hole and extends to a greater radius at the equator. A black hole with modest angular momentum has an ergosphere with a shape approximated by an oblate spheroid, while faster spins produce a more pumpkin-shaped ergosphere. The equatorial (maximal) radius of an ergosphere is the Schwarzschild radius, the radius of a non-rotating black hole. The polar (minimal) radius is also the polar (minimal) radius of the event horizon which can be as little as half the Schwarzschild radius for a maximally rotating black hole.