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 $5.36$. It is located at a distance of 8.165 kpc from the Sun, and 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 $6.9 arcsecond$, a half-mass radius of $0.54 arcminute$, and a tidal radius of $1.67 arcminute$. 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 $21.5 arcminute$ of the core, of which a dozen are long period variables. A millisecond pulsar was discovered in 1990.