ABOUT BLACK HOLES: A black hole forms when a massive star has used up all its fuel. The reason the sun and other stars emit light is because trillions of nuclear reactions are taking place at their cores. With core temperatures of millions of degrees, hydrogen atoms can convert into helium atoms, emitting radiation in the process. At some point, however, all the atoms are used up and no more nuclear fusion can take place. Without that outward counter-force to the pull of gravity, a star collapses inward, eventually reaching a point where the attractive gravitational force is so strong, not even light can escape. No one has ever observed the center of a black hole; until quite recently, such objects only existed in theory. But scientists surmise that a black hole has at its center an infinite density and an infinite gravitational field, as well as infinite entropy, which means no further change can take place. This is known as a "singularity." The event horizon of a black hole is not so much a physical surface as the theoretical point of no return for any object that gets caught in the black hole's powerful gravitational field.