Why can a black hole not be seen directly?

A black hole is a place in space where gravity pulls so strongly that nothing can get out, not even light. The pull is so great because a large amount of matter has been squeezed into a very small space. This can happen at the end of a star's life, when a dying star collapses.

Since no light can escape, a black hole cannot be seen directly. Scientists find black holes in another way: with special instruments on space telescopes, they watch how gas and stars behave close to one. Material swirling round an invisible point, or stars moving as if pulled by something unseen, can give a black hole away.

Black holes come in a range of sizes. The smallest, called primordial black holes, may be as small as a single atom but as heavy as a large mountain. The most common medium-sized ones are called stellar black holes. One of these can hold up to twenty times the mass of the Sun, packed into a ball only about sixteen kilometres across, and there may be dozens of them in our own galaxy, the Milky Way.

The largest are the supermassive black holes. They have the mass of more than a million Suns and would fill a ball about the size of the solar system. Scientists believe that almost every large galaxy has one at its centre.