A cluster of galaxies listed in the Abell Catalogue. To appear in the catalogue, a cluster must satisfy selection criteria which include containing more than 50 galaxies and having a dense concentration (richness). The clusters are classified as regular (R) or irregular (I) in appearance, ranked in increasing richness from 1 to 5, and increasing distance from 1 to 6. The approximate frequency of Abell clusters is one per 2.4 × 105 cubic megaparsecs.