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Helmi, A; van Leeuwen, F; McMillan, P J; Massari, D; Antoja, T; Robin, A; Lindegren, L; Bastian, U (2018). "Gaia Data Release 2: Kinematics of globular clusters and dwarf galaxies around the Milky Way". Astronomy and Astrophysics. 616. Gaia Collaboration: A12. arXiv:1804.09381. Bibcode:2018A&A...616A..12G. doi:10.1051/0004-6361/201832698. S2CID260496632.
Marín-Franch, Antonio; Aparicio, Antonio; Piotto, Giampaolo; Rosenberg, Alfred; Chaboyer, Brian; Sarajedini, Ata; Siegel, Michael; Anderson, Jay; Bedin, Luigi R.; Dotter, Aaron; Hempel, Maren; King, Ivan; Majewski, Steven; Milone, Antonino P.; Paust, Nathaniel; Reid, I. Neill (2009). "The ACS Survey of Galactic Globular Clusters. VII. Relative Ages". The Astrophysical Journal. 694 (2): 1498. arXiv:0812.4541. Bibcode:2009ApJ...694.1498M. doi:10.1088/0004-637X/694/2/1498. S2CID118407466.
Cudworth, Kyle M.; Rauscher, Bernard J. (1987). "Astrometry and Photometry in the Globular Cluster System M2". The Astronomical Journal. 93: 856. Bibcode:1987AJ.....93..856C. doi:10.1086/114369.
Stobie, R. S. (1971). "On the Difference Between the Oosterhoff Types i and II Globular Clusters". The Astrophysical Journal. 168: 381. Bibcode:1971ApJ...168..381S. doi:10.1086/151094.
Helmi, A; van Leeuwen, F; McMillan, P J; Massari, D; Antoja, T; Robin, A; Lindegren, L; Bastian, U (2018). "Gaia Data Release 2: Kinematics of globular clusters and dwarf galaxies around the Milky Way". Astronomy and Astrophysics. 616. Gaia Collaboration: A12. arXiv:1804.09381. Bibcode:2018A&A...616A..12G. doi:10.1051/0004-6361/201832698. S2CID260496632.