Abell, George O. (1955). “Globular Clusters and Planetary Nebulae Discovered on the National Geographic Society-Palomar Observatory Sky Survey”. Publications of the Astronomical Society of the Pacific67 (397): 258. Bibcode: 1955PASP...67..258A. doi:10.1086/126815.
Abell, George O. (1955). “Globular Clusters and Planetary Nebulae Discovered on the National Geographic Society-Palomar Observatory Sky Survey”. Publications of the Astronomical Society of the Pacific67 (397): 258. Bibcode: 1955PASP...67..258A. doi:10.1086/126815.