NGC 6535

NGC 6535 is a globular cluster of stars located at a distance of 22,200 light years from Earth in the equatorial constellation of Serpens, and is listed in the New General Catalogue. Its discovery is usually attributed to astronomer John Russell Hind in 1852.^[6][7] However, Wolfgang Steinicke has uncovered evidence that William Herschel's first discovery was actually NGC 6535, which he observed on 24 August 1780.^[8]

NGC 6535
Observation data (J2000 epoch)
Class	XI[citation needed]
Constellation	Serpens
Right ascension	18h 03m 50.51s[1]
Declination	−00° 17′ 51.5″[1]
Distance	22 kly (6.8 kpc)[2]
Apparent magnitude (V)	9.85[3]
Physical characteristics
Mass	(2.21±0.78)×104[4] M☉
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = −1.95±0.04[4] dex
Estimated age	12.75 Gyr[4]
Other designations	NGC 6535, C 1801-003, Glc 83[5]
See also: Globular cluster, List of globular clusters

This is a halo globular cluster located close to the galactic bulge^[2] at a distance of 13 kly (3.9 kpc) from the Galactic Center.^[4] It has an estimated age of 12.75 billion years.^[4] For a object of its type, NGC 6535 has a relatively low mass of 2.2×10⁴ times the mass of the Sun.^[4] The core radius is 2.3 ly (0.71 pc) and the tidal radius is 50 ly (15.3 pc).^[2] It has an unusually high mass-to-light ratio of 11.^[4]

The cluster is suspected to harbour an intermediate-mass black hole in its center.^[2] Unusually for a low-mass globular cluster, it has multiple generations of stars.^[4] Rather small and sparse for a globular cluster,^[9] this cluster contains no known RR Lyrae variables, which is unusual for a globular cluster. Nine blue stragglers have been discovered.^[10]