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