Messier 43 — Emission Nebula in Orion

M43 (also known as De Mairan's Nebula) is an emission nebula and HII region in the constellation Orion, located about 1,600 light-years from Earth. It is actually part of the Great Orion Nebula complex, separated from the main body of M42 by a dark lane of dust. M43 spans about 6 light-years and is illuminated by the young massive star NU Orionis (HD 37061), a B0.5 V type star with a surface temperature of about 30,000 K. Despite being cataloged separately, M43 is physically connected to M42 and shares the same parent molecular cloud. The nebula has a distinctive comma or fan shape.

Located immediately northeast of M42, separated by a conspicuous dark lane. It is visible in binoculars as part of the M42 complex but can be distinguished as a separate bright patch with a star embedded in it. In a 4-inch telescope at 50-80x, M43 appears as a round, bright nebulosity surrounding its central illuminating star, clearly separated from M42 by the dark dust lane. An OIII or UHC filter enhances the nebular extent. At higher magnifications, the uneven brightness and shape become more apparent. Best observed from November through March along with M42.

Discovered by Jean-Jacques Dortous de Mairan in 1731, hence its alternate name 'De Mairan's Nebula.' Charles Messier cataloged it separately from M42 in 1769, recognizing it as a distinct nebulous region. William Herschel later observed that M43 and M42 were connected despite the dark lane between them. Modern studies confirm that M43 is a blister on the near surface of the Orion Molecular Cloud, ionized by a single hot star.