Mayall II (M31 G1) 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 Andromeda's galactic core, and is the brightest (absolute magnitude) globular cluster in the Local Group, having an apparent magnitude of 13.7. G1 is considered to have twice the mass of Omega Centauri. G1 may contain a central, intermediate-mass (∼ 2×104 M⊙) black hole.

It was first identified as a possible globular cluster by Nicholas Mayall & O.J. Eggen in 1953 using a Palomar 48-inch 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 [bad word] globular cluster, but is actually the galactic core that remains of a dwarf galaxy consumed by Andromeda