When a star with a mass greater than about four times that of the sun collapses even the neutrons cannot stop the force of gravity. There is nothing to stop the contraction, and the star collapses forever. The material is so dense that nothing—not even light—can escape. The American physicist John Wheeler (1911–2008) gave this phenomenon the name “black hole” in 1967. Since no light escapes from a black hole, it cannot be observed directly. However, if a black hole existed near another star, it would draw matter from the other star into itself and, in effect, produce X rays. In the constellation of Cygnus, there is a strong X-ray source named Cygnus X–1. It is near a star, and the two revolve around each other. The unseen X-ray source has the gravitational pull of at least ten suns and is believed to be a black hole. Another type of black hole, a primordial black hole, may also exist dating from the time of the Big Bang, when regions of gas and dust were highly compressed. Recently, astronomers observed a brief pulse of X rays from Sagittarius A, a region near the center of the Milky Way galaxy. The origin of this pulse and its behavior led scientists to conclude that there is probably a black hole in the center of our galaxy.

There are four other possible black holes: a Schwarzschild black hole has no charge and no angular momentum; a Reissner-Nordstrom black hole has charge but no angular momentum; a Kerr black hole has angular momentum but no charge; and a Kerr-Newman black hole has charge and angular momentum.