An intensely interesting globular cluster due to its unusual metallicity and possible home to an intermediate mass black hole, Mayall II or G1 is part of the Andromeda galaxy. It has been proposed and widely accepted that this cluster may very well be the remnant nucleus of a dwarf galaxy which had its outer layers pulled away and incorporated into the Andromeda galaxy. The cluster currently resides in Andromeda's outer reaches, which gives us a very good vantage point to see it without a lot of unrelated stars mingling in around it.
This is some of the finest data I've ever had the pleasure to work with, with many exposures combined to create a very smooth noise floor and incredible signal to noise ratio. Virtually everything you see is either a star or a background galaxy, with cosmic rays and instrumental artifacts almost completely removed. If you zoom in on the cluster, you can see even some very faint members of the cluster resolved. Exquisite.
Filters used include near-infrared and near-ultraviolet, resulting in very colorful stellar populations. Aside from the obvious brightest stars, many of the small, very red sources are actually foreground stars which may easily be mistaken as part of the cluster. I only know this because around 8 years elapsed between some of the exposures, revealing proper motions of several stars. I ended up compensating for this by nudging them into place for the composition. I also had to deal with some misaligned diffraction spikes, which involved carefully picking them up and rotating them to match with the others. You may still be able to see some misalignment if you look closely.
Data from the following two proposals were used to create this image:
North is 37.00° counter-clockwise from up.
Red: WFC3/UVIS F814W
Orange: WFC3/UVIS F606W
Cyan: WFC3/UVIS F438W
Blue: WFC3/UVIS F336W