Sandage, Allan; Katem, Basil; Kristian, Jerome (1968). "An Indication of Gaps in the Giant Branch of the Globular Cluster M15". Astrophysical Journal. 153: L129. Bibcode:1968ApJ...153L.129S. doi:10.1086/180237.
Arp, Halton C.; Baum, William A.; Sandage, Allan R. (1953). "The color-magnitude diagram of the globular cluster M 92". Astronomical Journal. 58: 4. Bibcode:1953AJ.....58....4A. doi:10.1086/106800.
Sandage, Allan; Katem, Basil; Kristian, Jerome (1968). "An Indication of Gaps in the Giant Branch of the Globular Cluster M15". Astrophysical Journal. 153: L129. Bibcode:1968ApJ...153L.129S. doi:10.1086/180237.
Arp, Halton C.; Baum, William A.; Sandage, Allan R. (1953). "The color-magnitude diagram of the globular cluster M 92". Astronomical Journal. 58: 4. Bibcode:1953AJ.....58....4A. doi:10.1086/106800.