Mayall II (also designated G1, M31-G1, NGC-224-G1 or Andromeda’s Cluster) is a massive, slightly elliptical globular star cluster of approximately 42.5 light-years across orbiting the Andromeda Galaxy (M31). It is located about 130,000 light-years from Andromeda’s galactic core and 2.36 million light-years from Earth in the constellation Andromeda, but it is moving toward us at 332 kilometers per second.
It is the brightest globular cluster in the Local Group (a group of about 20 nearby galaxies, including the Milky Way). Mayal II consists of at least 300,000 very old stars — one million according to other sources — with an estimated mass of some 10 million solar masses. The cluster may contain a central, intermediate-mass black hole of about 20,000 solar masses. The age of Mayal II is probably 12 billion years!
A glimpse into the cluster’s finer details allow astronomers to see its fainter helium-burning stars whose temperatures and brightnesses show that this cluster in Andromeda and the oldest Milky Way clusters have approximately the same age. These clusters probably were formed shortly after the beginning of the Universe, providing astronomers with a record of the earliest era of galaxy formation.
Because of the widespread distribution of heavier elements, indicating multiple star generations and a large stellar creation period, many contend that Mayal II is not a true globular cluster, but that it is actually the galactic core that remained of a dwarf galaxy after it was consumed by Andromeda.
Seen from Earth, Mayal II lies almost in the middle of two bright foreground stars.
This colour picture is assembled from separate images taken with the Hubble Space Telescope in visible and near-infrared wavelengths. Image Credit: Michael Rich, Kenneth Mighell, and James D. Neill (Columbia University), Wendy Freedman (Carnegie Observatories), and NASA