^Shapley, Harlow; Sawyer, Helen B., A Classification of Globular Clusters, Harvard College Observatory Bulletin, August 1927,849 (849): 11–14,Bibcode:1927BHarO.849...11S.
^Lee, Jae-Woo, Multiple Stellar Populations of Globular Clusters from Homogeneous Ca–CN–CH Photometry. V.cn′JWL as a SurrogatecnJWL Index and NGC 6723, The Astrophysical Journal, 2019,883 (2): 166,S2CID 201070324,arXiv:1908.06670,doi:10.3847/1538-4357/ab3d34
Robert Burnham Jr,Burnham's Celestial Handbook: An observer's guide to the universe beyond the solar system, vol 3, p. 1558