A black hole is a body in space with gravity so strong that not even light can escape from it. Thus it is invisible and cannot be observed directly. In the early 1900s, scientists Albert Einstein and Karl Schwarzschild predicted the existence of black holes. Not until the 1970s, though, did astronomers have strong evidence of their existence.

Observing black holes

Since black holes let out no light, they cannot be seen directly. However, astronomers can notice the effects of their powerful gravity on other matter. For example, as a black hole draws matter toward it, the matter becomes heated. Just before the matter enters the black hole and disappears forever, it gives off large amounts of X rays.

Origins of black holes

Astronomers think that most black holes are formed when certain stars come to the end of their lives. For most of their lives, stars remain a constant size because they contain a balance of forces. Hydrogen atoms in the sun's core are constantly going through a process called fusion. As a result of fusion, heat is generated and that expands the star outward, while the force of gravity pulls it in. Eventually, in billions of years, the star uses up all of its hydrogen atoms and the fusion process stops. However, gravity continues to pull the star in on itself. Some stars collapse to a certain point and then stop, leaving a very dense remnant.

When a large star collapses, however, the process never stops. All the particles in the star are attracted to each other and pulled toward a central point called a singularity. This singularity is the center of a black hole. It is surrounded by what astronomers call an event horizon. An object attracted by gravity into a black hole would cross the event horizon and be lost forever, because in order to escape from a black hole objects would have to be able to travel faster than light.

Some black holes may have been formed from the collapse of large amounts of gas between stars rather than from the death of a star.