Messier catalog [key], systematic list of nebulae and star clusters. A first list, compiled and published in 1771 by Charles Messier, contained 45 objects. The final list, published in 1784, contained 103 objects; some of these were later removed from the list. Of the remaining objects, about 50 are extragalactic nebulae, i.e., galaxies. Designations from Messier's catalog are frequently used to refer to the brighter nebulae and star clusters; for example, M31 is the Andromeda Galaxy, M1 the Crab Nebula, M42 the Great Nebula in Orion, and M45 the Pleiades.
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