The photon sphere is a spherical boundary of zero thickness in which
photons that move on tangents to that sphere would be trapped in a circular orbit about the black hole. For non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. Their orbits would be dynamically unstable, hence any small perturbation, such as a particle of infalling matter, would cause an instability that would grow over time, either setting the photon on an outward trajectory causing it to escape the black hole, or on an inward spiral where it would eventually cross the event horizon.[74]
While light can still escape from the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. Hence any light that reaches an outside observer from the photon sphere must have been emitted by objects between the photon sphere and the event horizon.[74]
Other compact objects , such as neutron stars , can also have photon spheres. [75] This follows from the fact that the gravitational field external to a spherically-symmetric object is governed by the Schwarzschild metric , which depends only on the object's mass rather than the radius of the object, hence any object whose radius shrinks to smaller than 1.5 times the Schwarzschild radius
