Black holes are objects that are so massive that not even light can escape them, at least in theory. Many black holes are created when supermassive stars die and collapse in on themselves, and they grow as their immense gravitational pull vacuums up the gas and dust around them.
Smaller black holes can be the size of cities but have three times the mass of the sun, while other types of black holes can be the size of the sun but have billions of times its mass. Scientists believe that these larger black holes form the centers of galaxies, and they speculate that they might be created by the consolidation of a group of black holes or by the collapse of gas clouds or star clusters.
Scientists believe that there may also be medium-sized black holes, formed when star clusters collapse together. However, like all black holes, these intermediate black holes are detect because they gobble up light itself.
Black holes have three layers: the singularity, the inner event horizon and the outer event horizon. The event horizon is the boundary past which nothing can escape, and the singularity is the point in space-time that contains the black hole itself.