Just about everyone has heard of black holes. But many nonscientists are not exactly sure what these bizarre objects are and what they are capable of doing. This is partly because their name can be somewhat misleading. A black hole is not simply an empty hole, or void, in space, but instead a cosmic entity having very substantial mass. (Mass is the measurable quantity of matter possessed by physical objects.) A black hole does have a hole, or tunnel, as part of its structure; but unlike a void, it possesses a number of physical properties as well, some of which can be measured by human instruments.

For example, a black hole's large mass generates an equally large gravitational field, which scientists can detect. Gravity is a force or property of all matter that attracts one object...