Andromeda XXI is a moderately bright dwarf spheroidal galaxy about away from the Sun in the constellation Andromeda. It is the fourth largest Local Group dwarf spheroidal galaxy.
This large satellite of the Andromeda Galaxy has a half-light radius of nearly 1 kpc.
The discovery arose from the first year data of a photometric survey of the M31/M33 subgroupings of the Local Group by the Pan-Andromeda Archaeological Survey. This survey was conducted with the Megaprime/MegaCam wide-field camera mounted on the Canada-France-Hawaii Telescope.
Andromeda XXI appears as a spatial overdensity of stars. It has red giant branches at the distance of M31/M33, and follows metal-poor, =-1.8 when plotted in a color-magnitude diagram.
Although moderately bright, it has low surface brightness. This indicates that numerous relatively luminous M31 satellites remain undiscovered.