M42 definitions

(astronomy) The Orion Nebula; a diffuse nebula located at a distance of approximately 1600 light years from Earth, in the direction of the constellation Orion.
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Origin of m42

From Charles Messier, who first published Nebulae and Star Clusters in 1774 as a listing of visible objects in the night sky that were not stars. Objects in Messier's catalog are numbered, and the letter M (for Messier) is prepended to these numbers.