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 semimajor 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. For other uses, see sphere (disambiguation). ...
Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. ...
A black hole is a concentration of mass great enough that the force of gravity prevents anything from escaping it except through quantum tunnelling behaviour (known as Hawking Radiation). ...
It has been suggested that this article or section be merged into gravity. ...
In physics, the photon (from Greek Ï†Ï‰Ï‚ phos, meaning light) is the quantum of the electromagnetic field, for instance light. ...
In mathematics, the word tangent has two distinct, but etymologically related meanings: one in geometry, and one in trigonometry. ...
In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ...
In classical geometry, a radius of a circle or sphere is any line segment with one endpoint on the circle (i. ...
The Schwarzschild radius or gravitational radius is a characteristic radius associated with every mass. ...
Video Release: December 15, 1998 This article is on the film Event Horizon. ...
The semimajor axis of an ellipse In geometry, the term semimajor axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolas. ...
External links
 Step by Step into a Black Hole
 Virtual Trips to Black Holes and Neutron Stars
