Which Milky Way nebula is cataloged as Messier 42?

The Milky Way nebula cataloged as Messier 42 is the Orion Nebula.

Known as M42 or NGC 1976, the Orion Nebula is a bright diffuse nebula in the constellation Orion. It appears as the fuzzy middle “star” in Orion’s Sword, just south of Orion’s Belt, and is visible to the naked eye under reasonably dark skies. It lies about 1,267 light-years from Earth and spans roughly 25 light-years.

M42 is a stellar nursery: a cloud of gas and dust where new stars are forming. At its heart is the very young Trapezium Cluster, whose four brightest stars create a recognizable pattern. Ultraviolet light from these massive stars ionizes nearby hydrogen gas, producing much of the nebula’s characteristic glow.

The Orion Nebula is sometimes confused with nearby M43, which is a separate but connected nebular region, or with the Horsehead Nebula elsewhere in the larger Orion molecular-cloud complex. The nebula’s diffuse appearance was recorded telescopically by Nicolas-Claude Fabri de Peiresc on November 26, 1610.