Messier number

Etymology
From (1730-1817), the French astronomer who first published his Catalogue of Nebulae and Star Clusters in 1774.

Noun

 * 1)  A  number used to identify any one of more than one hundred galaxies, nebulae, and other  objects visible in the night sky; the designation formed by the surname  or letter  being prepended to the number, as in Messier 31 or M31 (the Andromeda Galaxy).