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Rotating black holes are thought to be formed in the collapse of a massive rotating star or from the collapse a collection of stars with an average non-zero angular momentum. Most stars rotate and therefore it is expected that most black holes in nature are rotating black holes. Rotating black holes can produce large amounts of energy at the expense of its rotational energy. In that case a rotating black hole gradually reduces to a Schwarzschild black hole, the minimum configuration from which no further energy can be extracted. The formation of a rotating black hole is thought to be observed as the emission of
gamma ray bursts.A black hole in general is surrounded by a spherical surface, the event horizon situated at the Schwarzschild radius, where the escape velocity is equal to the velocity of light. Within this surface, no observer/particle can maintain itself at a constant radius. It is forced to fall inwards and therefore sometimes called the static limit.
A rotating black hole has the same static limit at the Schwarzschild radius but there is an additional surface outside the Schwarzschild radius, which can be intuitively characterized as the sphere where "the rotational velocity of the surrounding space" is dragged along with the velocity of light. Within this sphere the dragging is greater than the speed of light. So, within this sphere, no observer/particle can maintain itself in a non-rotating orbit, but is forced to become co-rotated.
The region outside the event horizon but inside the sphere where the rotational velocity is the speed of light, is called the ergosphere (from Greek ergon meaning work). Particles falling within the ergosphere are forced to rotate faster and thereby gain energy. Because they are still outside the event horizon, they may escape the black hole. The net process is that the rotating black hole emits energetic particles at the cost of its own total energy. The possibility of extracting spin energy from a rotating black hole was first proposed by the mathematician Roger Penrose in 1969. The process is called the Penrose-process. Rotating black holes in astrophysics are a potential source of large amounts of energy and is used to explain energetic phenomenae, such as gamma ray bursts.
A rotating black hole is very different from a Schwarzschild black hole in that the spin of the black hole will cause the creation of two event horizons, an inner one and an outer one. As the spin increases, the inner event horizon moves outward, and the outer one moves inward. If the spin is great enough, the two will eventually merge and shrink towards the singularity. With no event horizons to hide it from the rest of the universe, the black hole cease to be a black hole, but a naked singularity.
Due to the two event horizons, a rotating black hole is also required to have two photon sphere s, an inner and an outer one. The greater the spin of the black holes is the farther from each other the photon spheres move. A beam of light travelling in a direction opposite to the spin of the black hole will be caught in the outer photon sphere. A beam of light travelling in the same direction as the black hole's spin will caught in the inner photon sphere.