The region just outside the event horizon of a rotating black hole (i.e. a Kerr black hole), within which an observer is forced to rotate with the black hole, and cannot remain stationary with respect to the rest of the Universe. The outer boundary of the ergosphere is the static limit. In a non-rotating Schwarzschild black hole, the event horizon and stationary limit coincide. The ergosphere derives its name from the fact that energy can be extracted from the black hole in this region via the Penrose process (the prefix ‘ergo’ means ‘work’).