Paust, Nathaniel E. Q. et al. (February 2010), “The ACS Survey of Galactic Globular Clusters. VIII. Effects of Environment on Globular Cluster Global Mass Functions”, The Astronomical Journal139 (2): 476–491, Bibcode: 2010AJ....139..476P, doi:10.1088/0004-6256/139/2/476.
Shapley, Harlow; Sawyer, Helen B. (August 1927), “A Classification of Globular Clusters”, Harvard College Observatory Bulletin (849): 11–14, Bibcode: 1927BHarO.849...11S.
Paust, Nathaniel E. Q. et al. (February 2010), “The ACS Survey of Galactic Globular Clusters. VIII. Effects of Environment on Globular Cluster Global Mass Functions”, The Astronomical Journal139 (2): 476–491, Bibcode: 2010AJ....139..476P, doi:10.1088/0004-6256/139/2/476.