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5. THE SIGNIFICANCE OF TYPES I AND II

According to the criteria of Table 1, Type I is defined by a centrally located cD galaxy - that is, by an outstandingly large, luminous galaxy having an elliptical-like nucleus surrounded by an extensive envelope. The most striking characteristic of such galaxies is their extremely large linear dimensions. At Type II, the brightest member galaxies are smaller in linear dimensions than at Type I; however, they may be comparable in luminosity with some cD galaxies in Type I clusters. For example, in the Coma cluster (A1656) Abell has found (private communication) that the brightest member galaxy stands out in integrated luminosity from its cluster background by as large an amount as does the cD galaxy NGC 6166 from its Type I cluster (A2199).