What is X-ray astronomy? Light, infrared and X-rays are all forms of electromagnetic radiation. We can sense visible light and infrared, but we cannot see or feel X-rays. No one knew of their existence until 1895. Since they can pass through all but the densest materials, X-ray machines are used in hospitals to see the bones and organs of the human body. However, large doses of X-rays are very dangerous and can cause cancer.

German physicist Wilhelm Konrad Roentgen (1845-1923) discovered X-rays in 1895

Nuclear explosions produce the extremely high temperatures needed to generate X-rays

When we look in a mirror, our eyes detect the light reflected from its shiny surface. Unfortunately, normal mirrors are useless for reflecting X-rays. Instead, they have to be focussed by allowing them to skim off a series of curved mirrors, rather like pebbles bouncing off a smooth pond. However, even this is very hard to achieve, and only a small fraction of the X-rays entering a telescope can be reflected off a single set of curved mirrors, so it is difficult to get a detailed picture of X-rays from space.

In the European Space Agency's X-ray Multi-Mirror (XMM) satellite, due for launch in late 1999, this problem will be solved by packing 58 mirrors into each of the satellite's three telescopes. These will capture and focus some 60 per cent of the incoming X-rays, allowing us to see the X-ray Universe in unprecedented detail. By the end of its 10 year life, the observatory will have revealed many of the secrets of such exotic objects as supernovae, X-ray binary stars, active galaxies and distant quasars (quasi-stellar objects).

A quasar - a gigantic powerhouse driven by a massive black hole