The Messier Objects are a set of over 100 astronomical objects first listed by French astronomer Charles Messier in 1771. Messier was a comet hunter, and was frustrated by objects which resembled but were not comets, so he compiled a list of them, in collaboration with his assistant Pierre Méchain, to avoid wasting time on them. The number of objects in the catalog reached 103 during his lifetime but a few more thought to have been observed by Messier have been added by other astronomers over the years.
The first edition covered 45 objects numbered M1 to M45. The total list published by Messier finally contained 103 objects, but the list "got an independent life" by successive additions by other astronomers, motivating the additions by side notes in Messier’s and Mechain’s texts indicating that either of them knew of the objects. The first such addition came from Nicolas Camille Flammarion in 1921, who added Messier 104 after finding Messier’s side note in his 1781 edition exemplar of the catalogue. M105 to M107 were added by Helen Sawyer Hogg in 1947, Messier 108 and Messier 109 by Owen Gingerich in 1960, and Messier 110 by Kenneth Glyn Jones in 1967.
Hidden Messier 111
For our purposes working with deep sky objects that remain hidden in the current list of Messier Objects that are listed to 110, our HGS manual lists Messier 111-Messier 120 as potential hidden deep sky objects that are identified by Guardian Krystal Star forces when incorporating these objects into an HGS session.