NGC 6539
Globular cluster in the constellation Serpens
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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.[6][7] This cluster is visible with a small amateur telescope, having an apparent visual magnitude of 9.6 and an angular size of 6.9″.[4] It is located at a distance of 26.63 kly (8.165 kpc) from the Sun,[3] and 10 kly (3.1 kpc) from the Galactic Center.[2]
| NGC 6539 | |
|---|---|
Globular cluster NGC 6539 | |
| Observation data (J2000 epoch) | |
| Class | X:[1] |
| Constellation | Serpens |
| Right ascension | 18h 04m 49.74s[2] |
| Declination | –07° 35′ 09.1″[2] |
| Distance | 26.63 kly (8.165 kpc)[3] |
| Apparent magnitude (V) | 9.6[4] |
| Apparent dimensions (V) | 6.9′[4] |
| Physical characteristics | |
| Absolute magnitude | −8.21[2] |
| Mass | 5.36×105[5] M☉ |
| Metallicity | [Fe/H] = −0.66[2] dex |
| Other designations | GCL 85 |
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]
