These are references to objects in the Catalog of Nebulae and Star Clusters, a list of 103 objects published by Charles Messier in 1774 through 1781. (The list has now been expanded to approximately 110 objects.)
Messier was a French astronomer who was particularly interested in searching for comets, and it is generally believed that his list started as an aide to this search: a list of fuzzy nebular objects that might be mistaken for comets but were not. By the time he published the final version, it clearly contained objects that could not be mistaken for comets, and had become a list of noteable and interesting objects within reach of the telescopes of that era.
The list now makes an excellent “hit parade” of targets for amateur astronomers in the Northern hemisphere because:
- Optics of the 1700s were primitive compared to today’s optics, so things Messier could see with difficulty are now easy targets for modern telescopes.
- Messier did not travel much – his objects are all observable from mid-Northern latitudes.
The list is not complete, and contains a few odd entries. For example
- It’s hard to imagine how he could have omitted the Double Cluster (NGC 869/884), which is easy to find and more impressive than some of the clusters in the catalogue.
- Certain objects were missing (e.g. M102) or seem to refer to relatively uninteresting objects (M40, a double star).