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)[3] from the Andromeda Galaxy's galactic core, and is the brightest[3] (by absolute magnitude) globular cluster in the Local Group, with an absolute visual magnitude of −10.94 and the luminosity of 2 million Suns.

Mayall II is considered to have twice the mass of Omega Centauri, and may contain a central, intermediate-mass (~ 2×104 M⊙) black hole.

[3] 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.

[3] 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 that remains of a dwarf galaxy consumed by Andromeda.