The Messier Catalog

The Messier catalog includes 110 deep-sky objects—including galaxies, nebulae, star clusters and more—cataloged by French astronomer Charles Messier . The catalog was originally published in 1774, in the Catalogue des Nébuleuses et des Amas d’Étoiles (“Catalogue of Nebulae and Star Clusters”).

Today, astronomers refer to the objects by their “M-codes”—M1 through M110 (or Messier 1 through Messier 110). Many of the objects are featured in other catalogs, such as the “New General Catalogue,” which has its own numeric system. And many objects are so famous that they also have proper or popularized names. For example, Messier 101 or M101 is also known as the Pinwheel Galaxy, and has a variety of designations including NGC 5457, UGC 8981, PGC 50063 and Arp 26.

Messier Catalog
The first catalog page of Messier’s third version from 1781 with the objects M 1 to M 5

The objects in the catalog are grouped and defined as follows::

  • Open cluster: Loose grouping of stars that formed from the same stellar nursery
  • Globular cluster: Region of our galaxy where hundreds of thousands of stars formed at the same time from the same dust cloud
  • Star cloud: Region where stars appear numerous and close together
  • Asterism: Small pattern or group of stars, which is not one of the 88 constellations recognized by the International Astronomical Union
  • Supernova remnant: Layers of matter that remain after the explosion of a giant star
  • Spiral galaxy: Twisted system of stars, dust, interstellar gas, and dark matter held together by gravity
  • Elliptical galaxy: Galaxy with a generally elliptical shape, and no apparent internal structure or spiral arms
  • Lenticular galaxy: Type of galaxy intermediate, between a spiral and elliptical galaxy 
  • Irregular galaxy: Galaxy that lacks a distinct shape, with neither a nuclear bulge nor any trace of spiral arm structure
  • Diffuse nebula: An extended interstellar cloud of dust, hydrogen, helium and other ionized gases, which has no well-defined boundaries
  • Planetary nebula: Type of emission nebula with an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives 

Messier’s catalog was compiled in collaboration with his assistant, Pierre Méchain. The first version of the catalogue, published in 1774, contained 45 not-yet-numbered objects, 18 of which were discovered by Messier.  The rest of the objects had been previously observed by other astronomers. 

Charles Messier
Messier's Drawings of M31/32/110 (1807)
Orion Nebula by Charles Messier
Orion Nebula (M42) - graphic by Charles Messier, 1771

By 1780, the catalogue had 70 objects, and by the next publishing in 1784 it contained 103 objects. Over time, objects were added by other astronomers using notes from Messier, and today the catalog stands at 110.

Twice a year, the most dedicated amateur astronomers try to observe all 110 objects from the Messier catalog in a single night. These events are known as Messier Marathons and require preparation, dedication and planning.