What is a Black Hole?

A black hole is a region of space where the gravitational pull is so strong that no matter can leave the black hole. According to Einstein's General Relativity, electromagnetic radiation (light) is bent by gravity. Near a black hole, gravity is so strong that light that falls into the black hole cannot escape (with an exception, see part 2). Thus, properly speaking, one cannot "see" a black hole. Its existence has to be deduced from other observations.

A black hole will have a "horizon"; beyond that point, matter or light that falls into the black hole cannot come back out. Near the horizon, nearby matter is accelerated and sucked into the black hole. Accelerating matter will emit great amounts of radiation. Radio jets, left picture, and visible light (right picture) show that near the center of the galaxy there is a powerful attractor of matter; likely this is a black hole.

What happens if a spaceship travels near or through the horizon of a black hole? Near the horizon, gravitational attraction varies enormously as the spaceship approaches the horizon; the front and back of the ship would feel a great difference in gravitational attraction that the ship would be torn apart.

Astronomers suspect that several galaxies have massive black holes in their centers; there are reasons to believe that the Milky Way, our own galaxy, might have a large black hole one as well.

Another candidate for a black hole is the quasar or quasi-stellar radio source. Quasars, although they are found at the "edge" of the observable universe, emit vast amounts of energy in the radio spectrum. It is suspected that this energy comes from matter falling into a large black hole.

Left: Image of galaxy NGC4261, 45 million light years from Earth. The orange part is radio signals represented in false color.

Right: Hubble's space telescope image of the same galaxy. It is suspected that there is a black hole at the center of this image.