Harrington, Philip S. (2010). "Medium-scope challenges". Cosmic Challenge: The Ultimate Observing List for Amateurs. Cambridge University Press. p. 241. ISBN9781139493680.
Ortolani, S.; Bica, E.; Barbuy, B. (1999), "Blue horizontal branch globular clusters towards the bulge: Terzan 9, NGC 6139 and NGC 6453", Astronomy and Astrophysics Supplement Series, 138 (2): 267–273, Bibcode:1999A&AS..138..267O, doi:10.1051/aas:1999275
Ortolani, S.; Bica, E.; Barbuy, B. (1999), "Blue horizontal branch globular clusters towards the bulge: Terzan 9, NGC 6139 and NGC 6453", Astronomy and Astrophysics Supplement Series, 138 (2): 267–273, Bibcode:1999A&AS..138..267O, doi:10.1051/aas:1999275
seds.org
spider.seds.org
"SEDS". spider.seds.org. Retrieved December 5, 2017.