The Messier objects are a set of 110 astronomical objects catalogued by the French astronomer Charles Messier in his Catalogue des Nébuleuses et des Amas d'Étoiles ("Catalogue of Nebulae and Star Clusters").
This catalogue of objects is one of the most famous lists of astronomical objects, and many Messier objects are still referenced by their Messier number.
The catalogue includes deep-sky objects that can be observed from Earth's Northern Hemisphere; many Messier objects are extremely popular targets for amateur astronomers.
Note that only 108 of the 110 Messier objects are visible in the Pro version. This is because two Messier objects are anomalous and hard to categorize:
- M40 is a binary star
- M24 is pseudo-cluster of stars (not an actual cluster of stars that are tied together gravitationally)
See this note for more info.