The upper limit on the mass of a neutron star is about 3 solar masses. Beyond that mass, the star can no longer support itself against its own gravity, and it must collapse. No known force can prevent the material from collapsing all the way to the point-like singularity, a region of extremely high density where the known laws of physics break down. Surrounding the singularity, at a distance of a few kilometres for a solar-mass object, is a region of space from which even light cannot escape from – a black hole. Astronomers believe that the most massive stars form black holes, rather than neutron stars, after they explode in a supernova.
Conditions in and near black holes cannot be described by Newtonian mechanics. A proper description involves the theories of relativity developed by Albert Einstein early in the twentieth century. Even relativity theory fails right at the singularity.
The "surface" of a black hole is the event horizon. At the event horizon, the escape velocity equals the speed of light. Within this distance, nothing can escape. Even photons passing too close to a black hole are deflected onto paths that cross the event horizon and become trapped.
Relativity theory describes gravity in terms of a warping, or bending, of space by the presence of mass. The more mass, the greater the warping. All particles – including photons - respond to that warping by moving along curved paths. A black hole is a region where the warping is so great that space folds back on itself, cutting off the interior of the hole from the rest of the universe.
To a distant observer, the clock on a spaceship falling into a black hole would show time diliation – it would appear to slow down as the ship approached the event horizon. The observer would never see the ship reach the surface of the hole. At the same time, light leaving the ship would be subject to gravitational redshift as it climbed out of the hole's intense gravitational field. Light emitted just at the event horizon would be redshifted to infinite wavelength. Both phenomena are predictions of the theory of relativity. However, the gravitational redshifts due to both the Earth and the Sun are very small, but have been detected experimentally.
Once matter falls into a black hole, it can no longer communicate with the outside. However, on its way in, it can form an accretion disk and emit X-rays just as in the neutron-star case. The best candidates for black holes are binary systems in which one component is a compact X-ray source. Cygnus X-1, a well-studied X-ray source in the constellation Cygnus, is a long-standing black hole candidate. Studies of orbital motions imply that the compact objects are too massive to be neutron stars, leaving black holes as the only logical alternative.