The Abell catalogue is a catalogue of approximately 4,000 galaxy clusters with at least 30 members, almost complete to a redshift of z = 0.2. It was originally compiled by the American astronomer George O. Abell in 1958 using plates from POSS, and extended to the southern hemisphere by Abell, Corwin and Olowin in 1987. The name "Abell" is also commonly used as a designation for objects he compiled in a catalogue of 86 planetary nebulae in 1966. The proper designation for the galaxy clusters is ACO, as in "ACO 13", while the planetary-nebula designation is the single letter A, as in "A 39".
Comprises such a huge amount of mass that the galaxy cluster noticeably curves spacetime around it, making it into a gravitational lens. Smaller galaxies behind the cluster appear distorted into long, warped arcs around the cluster’s edges.[7]
^Reichert G, Mason KO, Charles PA, Bowyer S, Lea SM, Pravdo S (Aug 1981). "Low energy X-ray emission from five galaxy cluster sources". Astrophys. J. 247: 803–12. Bibcode:1981ApJ...247..803R. doi:10.1086/159092.