The Andromeda Galaxy, Messier 31, M31. Also on this picture is Messier 32, M32 - the bright light left of the Andromeda Galaxy core - and Messier 110, M110 - the little galaxy in the lower right of the picture.
My 2023 reprocessed version of my 2021 Andromeda Galaxy data. A comparion of both versions is worth a look.