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The Coma cluster

Most of the galaxies in the universe are found in CLUSTERS bound together by gravitational forces. In the visible band optical photographs of the centre of the Coma cluster, show large numbers of galaxies close together on the sky. When we look in the same direction with an X-ray telescope the appearance is radically different as is shown in the accompanying ROSAT image. Instead of the individual galaxies we see a enormous cloud of hot gas filling the whole cluster. The gas itself was heated as it fell into the cluster under the influence of gravity, reaching temperatures of around 50 million degrees, guaranteeing emission at X-ray wavelengths.

By studying the X-ray emission from clusters of galaxies like Coma we can measure the amount of hot gas involved and hence deduce how much gravitational mass is needed. X-ray observations provide the best evidence we have that a large fraction of the gravitational mass must be in the form of "dark matter".

 
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