What is a black hole?

A black hole and its companion star

Fig. 1   A schematic diagram showing a black hole and its companion star in a binary system. Material from the companion rushes into the black hole along a sprial path, frictional force heats up the material to a high temperature, and high energy radiation is emitted.

A black hole is a celestial object of a very high density. The gravitational field surrounding it is so strong that all passing by materials are absorbed into it, even light cannot escape. Neither can materials inside a black hole escape from it. We know that an object with a very high initial velocity on earth can free itself from the earth's gravitational field and fly to the space. If the initial speed is high enough, the object can even get rid of the solar gravitational field and escape from the solar system. The launch of a satellite or a spacecraft with a rocket is based on this principle. The lowest velocity for an object to escape from the earth is set by the mass and radius of the earth. If the earth is compressed into a very small ball and its radius is smaller than a certain critical value, its gravitational field would become very large in its surrounding. Though light is a wave with the highest propagation speed in the universe, it would also be trapped by the earth and cannot escape. The earth would then become a blackhole.

According to the stellar evolution theory, black hole is the last stage in stellar evolution. It is called a "dead" star. As a black hole does not emit any material or radiation, we cannot observe it directly. Nevertheless, a black hole can combine with a neighbouring star to form a binary star system. Observing from the earth, this visible neighbouring star is like dancing the waltz with an invisible celestial object (see figure). The mass of this invisible object can be determined by studying the motion of the visible star. If the mass of this invisible object is very large, it is probably a black hole. Furthermore, the strong gravitational field of a black hole causes the gaseous materials of its neighbouring star to fall into it along a spiral track. Being compressed hastily, these materials become very hot, and thus X-ray and ray are emitted. These radiations we observed on earth are evidences of the existence of black holes.