Bajkova, A. T.; Bobylev, V. V. (2019), "Search for the evolutionary relationship between Galactic globular and open clusters using data from the Gaia DR2 catalogue", Monthly Notices of the Royal Astronomical Society, 488 (3): 3474–3481, arXiv:1907.10939, doi:10.1093/mnras/stz2061
Bajkova, A. T.; Bobylev, V. V. (2019), "Search for the evolutionary relationship between Galactic globular and open clusters using data from the Gaia DR2 catalogue", Monthly Notices of the Royal Astronomical Society, 488 (3): 3474–3481, arXiv:1907.10939, doi:10.1093/mnras/stz2061
Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin, 849 (849): 11–14, Bibcode:1927BHarO.849...11S.