The range of directions within which light can escape from the gravitational field of an object, centred on the vertical to the object's surface. The concept is of significance only for an object collapsing to become a black hole. Then, rays of light sent out at a large angle to the vertical are bent back to the object's surface by its strong gravitational field. As the object collapses, the cone angle will decrease, closing up entirely as the object reaches its event horizon.
Subjects: Astronomy and Astrophysics.