A black hole is an invisible region in space. It is invisible because no light can get out of it. Nothing can get out of a black hole. Scientists think that black holes were once large stars and that a black hole forms when a large star runs out of fuel. When it stops burning fuel, the star shrinks. Next, it explodes and throws off its outer layers. Then it shrinks more and more.

The material that made up the star shrinks down to a tiny point. That point is a black hole. It is hard to imagine how so much material can fit into a tiny point. This great amount of material is what makes a black hole's force of gravity so powerful that nothing can escape.

Astronomers cannot see black holes directly. However, they can see how black holes affect stars and other objects in space. For example, a black hole may attract gas from a star and heat it up. The heated gas would give off rays of energy called X rays. Scientists can use special telescopes to see these X rays. If scientists know where the X rays are, they can locate the black hole. How to cite this article: To cite this article, World Book recommends the following format: "Black Hole." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.