The New General Catalogue of Nebulae and Clusters of Stars (abbreviated as NGC) is a catalogue of deep-sky objects compiled by John Louis Emil Dreyer in 1888. The NGC contains 7,840 objects, known as the NGC objects. It is one of the largest comprehensive catalogues, as it includes all types of deep space objects, including galaxies, star clusters, emission nebulae and absorption nebulae.
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NGC 6539
NGC 6539, or GCL 85, is a globular cluster of stars in the constellation Serpens. It was discovered by Danish astronomer Theodor Brorsen in 1856. This cluster is visible with a small amateur telescope, having an apparent visual magnitude of 9.6 and an angular size of 6.9″. It is located at a distance of 26.63 kly (8.165 kpc) from the Sun, and 10 kly (3.1 kpc) from the Galactic Center. 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. 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. A millisecond pulsar was discovered in 1990.
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Wikipedia Page: NGC 6539
NGC 6539 at In-The-Sky website