Abell richness classes

Abell richness classes

[′ā·bəl ′rich·nəs ‚klas·əz]
(astronomy)
A scale of six categories of richness into which clusters of galaxies are classified, based on the number of galaxies observed in the cluster that are not more than 2 magnitudes fainter than the third-brightest member.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.