When heavy stars reach the end of their lives, they explode as supernovae, then collapse into dense, dark objects called black holes, from which nothing, not even light, can escape. Black holes are dynamic objects, continuing to grow by "eating," or absorbing the mass, of nearby stars, or by merging with other black holes. Because black holes are extremely dense, they can be small in size but high in mass.
Black holes can be observed by their interaction with neighboring stellar objects. Since a black hole swallows the mass of a nearby star by drawing it into the hole, increased acceleration in a star's velocity and motion can demonstrate that it has entered the gravitational pull of a black hole. Black holes can also be observed through measurements of star mass in binary star systems. A star system's mass can exceed the mass of its only visible star, suggesting the presence of a dark, highly massive companion.
Bright light emitted in the vicinity of a black hole can indicate its presence. As matter accelerates in its fall into the black hole, it heats to high temperatures, emitting intense light and gasses before it vanishes. The Hubble Space Telescope, for example, has revealed hot, bright light at the center of the Milky Way, where gasses at the galaxy's core are orbiting faster than accounted for by visible stars. This suggests that at the center of the galaxy resides a supermassive black hole.
Like light, the behavior of stellar gasses can indicate the presence of a black hole. Hot gasses can circle a black hole at speeds of up to 200,000 miles per second, confirming the existence of an invisible object drawing them in. Observation of the behavior of gasses around a black hole provides information about the orbital period and rotation of black holes, and the ways in which they expand.