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There is another property of clusters of galaxies which has some interest as a parameter for classification: the degree to which the brightest member stands out against the general cluster background. Such bright objects are clearly important astrophysically, as well as for problems of galactic structure.

We have therefore devised a five-part form classification which depends on the relative contrast of the brightest galaxy to others in each cluster. The notation is given in Table 1.

Table 1. Criteria and standard clusters

Type Description (Abell No.)

I Clusters containing a centrally located cD galaxy A2199, A2029
I-II Intermediate
II Clusters where brightest galaxy or galaxies are intermediate in appearance between class cD and the Virgo-type giant ellipticals A0194, A1656 (Coma), A2197
II-III Intermediate A0426 (Perseus), A0400
III Clusters containing no dominant galaxies. This type can be subdivided into III-E and III-S, according to the absence or presence of considerable numbers of bright spirals Virgo, A2065 (CrB)

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