Most of the clusters we describe are termed RICH CLUSTERS of galaxies. This implies a fairly high degree of central concentration of galaxies. Abell’s catalogue tabulates 2712 rich clusters, which are numbered in order of increasing right ascension preceded by the letter A: examples are the Coma and Perseus clusters, A426 and respectively . Abell clusters are mostly in the northern celestial hemisphere. Apart from the rich clusters, F. Z wicky and his colleagues have produced diagrams of the distribution of galaxies that reveal many more clusters that are not necessarily rich. The advent of survey material from observatories in the southern hemisphere means that similar, and even more detailed, catalogues of the southern regions are now available.

Abell classified his clusters according to richness and distance. In this scheme, the larger the richness and distance class the more centrally concentrated and distant is the cluster. He also differentiates in form between regular and irregular clusters. Distance classes range from near to extra-distant; distance estimates are based upon estimates of galaxy absolute magnitudes. Other classification schemes have been devised which inquire more into the brightness contrast between the galaxies, or the form of the cluster.

Clusters can also be classified by their radio or X-ray properties. For example some, and perhaps most, rich clusters contain extended radio sources as well as more complex radio sources associated with member galaxies. Many clusters are observed to emit X-rays. At present, however, these properties are ill-defined and no clear-cut classes are apparent.

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