Photon sphere

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A photon sphere is a spherical region of space surrounding extremely massive objects such as black holes. At a particular distance, the gravitational force is strong enough that photons approaching along tangents to the sphere will become trapped in an unstable orbit. For black holes, the photon sphere has a radius 1.5 times larger than the Schwarzschild radius (the radius of the event horizon). No orbit with a semi-major axis less than this distance is possible, but within the photon sphere constant acceleration will allow a spacecraft or probe to hover above the event horizon.

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