Although its interior is invisible, a black hole may reveal its presence through an interaction with matter that lies in orbit around it. A black hole can be perceived by tracking the movement of a group of stars that orbit its center. One may observe gas from a nearby star that has been drawn into the black hole. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes.
A black hole is the evolutionary endpoint of star at least 10 to 15 times as massive as the Sun. If a star that massive undergoes a supernova explosion, it will leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself, that is to say it will implode, collapsing to the point of zero volume and infinite density, thus creating what is known as a singularity. As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star. Any emitted photons are trapped, too, by the intense gravitational field. Because no light escapes after the star reaches this infinite density, it is called a black hole.
Supermassive black holes that contain hundreds of thousands to billions of solar masses are believed to exist in the center of most galaxies, including our own Milky Way. They are thought to be responsible for active galactic nuclei, and form either from the coalescence of smaller black holes, or by the accretion of stars and gas onto them. The largest known supermassive black hole is located in OJ 287 weighing in at 18 billion solar masses.