|Observation data (J2000 epoch)|
|Right ascension||00h 32m 46.51s|
|Declination||+39° 34′ 39.7″|
|Distance||2.52 ± 0.14 Mly (770 ± 40 kpc)|
|Apparent magnitude (V)||+13.81|
|Mass||1×107 M☉ (2×1037 kg)|
|Radius||21.2 ± 1.0 ly (6.5 ± 0.3 pc) (Half light radius rh) and tidal radius 263.2 ± 12.7 ly (80.7 ± 3.9 pc)|
|Estimated age||~ 12 Gyr|
|Other designations||SKHB 1, HBK 0-1|
Mayall II, also known as NGC-224-G1, SKHB 1, GSC 2788:2139, HBK 0-1, M31GC J003247+393440 or Andromeda's Cluster, is a globular cluster orbiting M31, the Andromeda Galaxy.
It is located 130,000 light-years (40 kpc) from the Andromeda Galaxy's galactic core, and is the brightest (by absolute magnitude) globular cluster in the Local Group, having an apparent magnitude of 13.81 in V band. Mayall II is considered to have twice the mass of Omega Centauri, and may contain a central, intermediate-mass (∼ 2×104 M⊙) black hole.
It was first identified as a possible globular cluster by American astronomers Nicholas Mayall and Olin J. Eggen in 1953 using a Palomar 48-inch (1.2 m) Schmidt plate exposed in 1948.
Because of the widespread distribution of metallicity, indicating multiple star generations and a large stellar creation period, many contend that it is not a true globular cluster, but is actually the galactic core[disambiguation needed] that remains of a dwarf galaxy consumed by Andromeda.