Clusters of galaxies display a wide variety of morphological forms, ranging from rich aggregates of thousands of members to the relatively poor groups, like the Local Group which contains only 17 to 20 known members, or even to double or triple systems if these can be classed as clusters. The smaller groups appear to be by far the most numerous, but at present there exist no reliable data on the actual relative numbers of rich clusters and poor groups. Enough is known, however, to classify the clusters into certain broad categories. For the purposes of the Catalogue of Galaxies and Clusters of Galaxies, Zwicky (Zwicky, Herzog, Wild, Karpowicz, and Kowal 1961-1968) classifies clusters as compact, medium compact, and open. He defines a compact cluster as one with a single pronounced concentration of galaxies, in which 10 or more galaxies appear (in projection) to be in contact; a medium compact cluster is one with a single concentration within which galaxies appear to be separated by several of their diameters, or in which there are several pronounced concentrations of galaxies; an open cluster is one without any pronounced peak of population, but which appears as a loose cloud of galaxies superposed on the general field.
A more recent classification of galaxian clusters was suggested by Morgan (1961) on the basis of his study of the 20 nearest clusters in Abell's (1958) catalog. He found that the clusters investigated could be divided into two categories, according to the types of galaxies encountered among their brightest members: (i) those containing appreciable numbers of galaxies of minor central concentration of light (late spiral and irregular galaxies); and (ii) those containing few or none of the latter.
The classifications assigned to clusters by the Morgan and Zwicky systems are not independent, but are strongly correlated. The main features of both systems are preserved by simply classifying clusters as regular or irregular. Although the demarcation between the two classes is not a sharp one, they do uniquely describe many of the morphological characteristics of most clusters. Regular clusters are all rich, having populations of the order of 103 or more in the interval of the brightest 6 magnitudes. They show high central concentration and marked spherical symmetry. Their memberships consist entirely, or almost entirely, of galaxies without conspicuous dust - E and S0 galaxies. Examples are the famous clusters in Coma and Corona Borealis (fig. 1) (Abell catalog numbers 1656 and 2065, respectively). Most of Zwicky's compact clusters and Morgan's type ii clusters belong to this class. Irregular clusters range from poor groups, like the Local Group, to relatively rich aggregates like the Virgo cluster or the Hercules cluster (fig. 2) (Abell number 2151). They contain little or no spherical symmetry, and no marked central concentration, although multiple condensations are often present. These clusters usually contain galaxies of all types, including appreciable numbers of late-type spirals and irregulars. To this class belong Zwicky's medium-compact and open clusters, and Morgan's type i clusters. The principal features of regular and irregular clusters are summarized in table 1; explanatory discussion follows.
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Figure 1. The regular cluster, number 2065 (Corona Borealis), photographed with the 200-inch telescope. Scale: 1 mm = 3.9". |
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Figure 2. The irregular cluster, number 2151 (Hercules), photographed with the 48-inch Schmidt telescope. Scale: 1 mm = 22.3". |
| Parameter | Regular Clusters | Irregular Clusters |
| Symmetry | Marked spherical symmetry | Little or no symmetry |
| Concentration | High concentration of members toward cluster center | No marked concentration to a unique cluster center; often two or more nuclei of concentration are present |
| Types of galaxies | All or nearly all galaxies in the first 3 or 4 magnitude intervals are elliptical and/or S0 galaxies. | All types of galaxies are usually present except in the poor groups, which which may not contain giant ellipticals. Late-type spirals and/or irregular galaxies present |
| Number of member galaxies in range of brightest 7 mag | Order of 103 or more | Order of 101 to 103 |
| Diameter (Mpc) | Order of 1-10 | Order of 1-10 |
| Presence of subclustering | Probably absent or unimportant | Often present. Double and multiple systems of galaxies common |
| Dispersion of radial velocities of members about mean for cluster | Order of 103 km s-1 | Order of 102-103 km s-1 |
| Mass derived from virial theorem (see §4) | Order of 1015
 
| Order of 1012-1014
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| Other characteristics | Cluster often centered about one or two giant elliptical galaxies | |
| Examples | Coma cluster (No. 1656); CrB cluster (No. 2065) | Local Group, M81 group, Virgo cluster, Hercules cluster (No. 2151) |