The brightest globular cluster in Messier 31: M31-G1 or "Mayall II"
As the headline already tells us, the brightest globular cluster in the Andromeda Galaxy is called "M31-G1". The "G" stands for the English term "globular".
But beside this designation this object carries additionally the name "Mayall II", because it was discovered among others by Nicholas Mayall in 1953 on
photographic plates.
Although G1 is classified as a globular cluster, it is not sure yet, if this is really such a cluster. But why is this so?
From the apparent magnitude of mv= 13.5mag and the known distance of 2.52 million lightyears the absolute magnitude of G1 can be calculated to Mv= -10.9mag.
This corresponds to 2.03 million solar luminosities. This would make it not only the brightest globular cluster in the entire local group, but with 2 to 3 times the
mass of "Omega Centauri" also by far the most massive. Its weight is estimated to be between 12 and 17 million solar masses, so it can be assumed that there are
about 20 to 30 million individual stars in G1. The core of "Mayall II" is extremely concentrated, so the stellar density in the center is so immensely high that the
globular cluster is on the verge of a core collapse. Moreover, the outer appearance of G1 is not roundish, but rather slightly oval with an ellipticity of 0.19. The
long axis is about 1.5x as long as the short one.
All these points raise doubts if "Mayall II" is really a globular cluster or if it is perhaps the remaining core of a dwarf galaxy. Because these small galaxies show
similar, partly even lower values for luminosity and mass. For this theory would also speak that in the center of the globular cluster is one of the few known medium
heavy black holes with 20,000 solar masses.
The core radius of G1 is given in a scientific paper from 2007 with 2.5 light years. This is the area where the surface brightness has dropped to half. The "Half Light
Radius" indicates the radius within which half of the total light is emitted. For G1, this value is 21.2 light-years - an enormously large value for a globular cluster. By
the way, this is also the radius we can use as a guide for visual observation. Stars in the vicinity of G1 are gravitationally bound to the cluster even up to a distance of
265 light-years, so the total diameter of G1 could be given as about 530 light-years.
