Black holes are among the strangest and most mysterious objects in the cosmos. Simply put, a black hole is the remains of a massive star that has burned out its nuclear fuel and collapsed into a point of zero volume under tremendous gravitational force. This point is its "singularity," where the pressure and density are infinite. It cannot be directly detected by any means, yet black holes are not only believed to exist, they may represent 90 percent of the content of the universe.

The name "black hole" was first proposed by American physicist John Wheeler (1943-) in 1969. It had a novel sound to it, was highly publicized, and became a favorite with the general public. Russian scientists came up with the term collapsar, which, though more descriptive of the object, never caught on.

The final end of a star is directly related to its mass. Any...