X-ray astronomy is a relatively new science as we have needed satellites to do it; these have only been around since the 1950′s. X-rays have a tendency to go through things compared to visible light, but they are also fairly easily absorbed – a few millimetres of bone or a few metres of air will stop them. This is good for them, as large doses of X-rays are dangerous and can cause cell mutations, and there is plenty of air between us and space. So they are lucky that the Earths’ atmosphere blocks X-rays from reaching the Earths’ surface. This is because the atmosphere contains water which is opaque to X-rays. If all the water were in the atmosphere were condensed onto the surface, it would form a layer 10 m deep.

However this does mean that they need to get into orbit to look at X-rays from space. The picture shows how different wavelengths of electromagnetic radiation (“light”) are absorbed by the atmosphere. Therefore we need satellites to be able to do any X-ray astronomy.The first attempts at X-ray astronomy were just to see if there was any form of X-ray radiation in space. The instruments were simple detectors carried aloft on rockets which then parachuted back down to Earth. They detected X-ray emission from the Sun. The surface of the Sun is relatively cool (5800K) and so doesn’t produce many X-rays, but is therefore a very good emitter of visible light. For an object to emit most of its light in X-rays its temperature has to be ~6,000,000 degrees.

For more information, please visit http://www-xray.ast.cam.ac.uk