This cluster is located in the outer parts of the Galactic bulge. There is a large dark cloud complex located in the fore of this cluster, causing reddening from extinction.[8] NGC 6539 has a core radius of 0.54′, a half-mass radius of 1.67′, and a tidal radius of 21.5′. Compared to other globular clusters, NGC 6539 is considered metal rich, indicating that it has a relatively higher abundance of elements more massive than helium. Thirteen variable stars have been detected within 7′ of the core, of which a dozen are long period variables.[9] A millisecond pulsar was discovered in 1990.[10]
References
^Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin, 849 (849): 11–14, Bibcode:1927BHarO.849...11S.
^ abBaumgardt, H.; Vasiliev, E. (2021), "Accurate distances to Galactic globular clusters through a combination of Gaia EDR3, HST, and literature data", Monthly Notices of the Royal Astronomical Society, 505: 5957, arXiv:2105.09526, Bibcode:2021MNRAS.505.5957B, doi:10.1093/mnras/stab1474.
^Baker, JoDee M.; et al. (January 2007), "Variable Stars in Metal-Rich Globular Clusters. III. NGC 6539", The Astronomical Journal, 133 (1): 139–146, Bibcode:2007AJ....133..139B, doi:10.1086/509638.
^D'Amico, N.; et al. (May 1990), Green, D. W. E. (ed.), "Millisecond Pulsar in NGC 6539", IAU Circular, 5013: 1, Bibcode:1990IAUC.5013....1D.