NGC 6522

NGC 6522 is a globular cluster of stars in the southern constellation of Sagittarius. It was discovered by German-British astronomer William Herschel on June 24, 1784. The cluster has an apparent visual magnitude of 8.3 and an angular diameter of 9.4′.^[3] It is located at a distance of 25.1 kly (7.7 kpc) from the Sun,^[2] and lies in the Milky Way's central bulge,^[7] about 2.0 kly (0.6 kpc) from the Galactic Center.^[1] The cluster is centered in a region of the sky known as Baade's Window.^[8] It is highly impacted by reddening due to interstellar dust and the view is heavily contaminated by field stars, making it more difficult identify members.^[9]

NGC 6522
Observation data (J2000 epoch)
Class	VI[citation needed]
Constellation	Sagittarius
Right ascension	18h 03m 34.08s[1]
Declination	−30° 02′ 02.3″[1]
Distance	25.1 kly (7.7 kpc)[2]
Apparent magnitude (V)	8.3[3]
Apparent dimensions (V)	9.4′[3]
Physical characteristics
Absolute magnitude	−7.67[1]
Mass	5.93×104[4] M☉
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –1.34[2] dex
Estimated age	12.0 Gyr[5]
Other designations	GCl 82, C 1800-300[6]
See also: Globular cluster, List of globular clusters

NGC 6522 is possibly the oldest star cluster in the Milky Way,^[10] with an age of more than 12 billion years.^[11][12] It is a core collapsed cluster with a core radius of 0.5′ and a 1.0′ half-light radius.^[9] The cluster formed four billion years before the Milky Way galactic bar appeared, and may have been confined to the bar for a period of time. At present it trails the bar in its orbit around the core.^[13]

This is a low mass globular cluster with an estimated 5.93×10⁴ times the mass of the Sun. Distinctive chemical abundances among the members indicate the cluster has multiple populations of stars, with the younger populations exhibiting pollution from earlier generations.^[4] Twenty variable stars have been identified as members of NGC 6522, consisting of eight RR Lyrae, three type II Cepheids, and nine long-period variable stars.^[9] Six pulsars have been discovered.^[14]

Gallery

• 
  The globular clusters NGC 6528 (lower left) is close to NGC 6522 (upper right)
• 
  The cluster is located in Baade's Window, a region of night sky that is not clouded by dust from the Milky Way.