Cambridge X-Ray Astronomy Group
What are X-rays?

X-rays are a form of light, but much more energetic than the light detected by our eyes. The energy of an X-ray photon (light particle) is ~1000 times that of a photon of visible light. They are part of the electromagnetic spectrum which includes visible light, radio waves, microwaves and infra red radiation. The picture below should make this a bit clearer.

The relation of the frequency of light to the temperature of an object. X-rays come from very hot, and hence exciting objects, which means that there is some extreme physics going on. (Image courtesy NASA)

What the picture shows is that X-rays are emitted from things that are really hot - millions of degrees. K stands for Kelvin - a temperature scale which has the same units as Celsius, but starts at absolute zero (-273o C). The shorter the wavelength (higher frequency) the higher the energy of the light.

X-rays are so energetic that they pass straight through many materials, which is why they are used in hospitals to image bones to check for breaks and fractures.

The discovery of X-rays

X-rays were discovered in 1895 by Wilhelm Röntgen, a German physics professor working in a laboratory in the Physical Institute of the University of Würzburg. For this discovery he won the first Nobel prize in physics in 1901. He named them "X" rays as he was not sure what they were. They are absorbed better by materials which are dense, and so, when used in hospitals, they are stopped more by the bone and any metal (e.g. dental fillings) then the fleshy parts of the body. The X-rays cause the film to be exposed, and so if they are blocked, the film remains dark, hence producing a shadow of the denser parts of the body.

Astronomical X-rays

Astronomical X-rays are usually imaged in a different way. In hospitals the image produced is a silhouette, whereas in astronomy, we image the source of the X-rays itself. Sometimes, however, there is an object which does get in the way and this then can appear as a shadow on the X-ray emission.

Astronomers classify X-rays by their energy in the following way:

0.1 keV1.0 keV10.0 keV100 keV1 MeV
"Soft""Hard"~Soft gamma-rays

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Published 1996-2005 by the Institute of Astronomy X-Ray Group. Last Modified on 2005-04-28. Comments to the xray-webmaster at