Harrington, Philip S. (2010). „Medium-scope challenges“. Cosmic Challenge: The Ultimate Observing List for Amateurs. Cambridge University Press. p. 241. ISBN9781139493680.
Boyles, J.; Lorimer, D. R.; Turk, P. J.; Mnatsakanov, R.; Lynch, R. S.; Ransom, S. M.; Freire, P. C. & Belczynski, K. (November 2011), "Young Radio Pulsars in Galactic Globular Clusters", The Astrophysical Journal742(1): 51, doi:10.1088/0004-637X/742/1/51, Bibcode: {{{bibcode}}}
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 Series138(2): 267–273, doi:10.1051/aas:1999275, Bibcode: {{{bibcode}}}
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Boyles, J.; Lorimer, D. R.; Turk, P. J.; Mnatsakanov, R.; Lynch, R. S.; Ransom, S. M.; Freire, P. C. & Belczynski, K. (November 2011), "Young Radio Pulsars in Galactic Globular Clusters", The Astrophysical Journal742(1): 51, doi:10.1088/0004-637X/742/1/51, Bibcode: {{{bibcode}}}
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 Series138(2): 267–273, doi:10.1051/aas:1999275, Bibcode: {{{bibcode}}}