Themes > Science > Astronomy > The Galaxies > Groups, Clusters, and Superclusters of Galaxies > The Mass Contained in Clusters


Clusters contain contain 1014 to 1015 solar masses. They must have masses of this size to explain how they are held together by gravity, but only about 1/10 of this mass can be identified with matter that is emitting visible light. This is known as the `missing mass problem'.

Hot X-ray Gases in Clusters

One source of this additional mass is gas between the galaxies in a cluster. Superposed on an optical picture of a group of galaxies in the adjacent figure is an X-ray image taken by ROSAT. The image shows confined hot gas (which produces X-rays) highlighted in false red color. In order to produce X-rays, the gases must have temperatures of 10-100 million degrees (Ref).

The presence of this confined gas indicates that the gravity exerted in groups and clusters of galaxies is larger than that expected from the visible mass in the galaxies, for otherwise the hot gas would have been expected to disperse long ago. Here is a discussion of such hot X-ray gases as observed in the Virgo Cluster.

Dark Matter

The hot X-ray gas is estimated have a mass comparable to the visible matter of the galaxies in such clusters, but this is still far too little matter (by a factor of 5 to 10) to account for the gravitational forces that are holding the clusters together. The matter that is seen neither in the visible spectrum nor the X-ray spectrum (but is "seen" by its gravitational influence) is termed dark matter. The nature of the dark matter is one of the great current mysteries in astronomy.


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