Messier 78 — Reflection Nebula in Orion

M78 is the brightest diffuse reflection nebula in the sky, located about 1,600 light-years away in the constellation Orion. It is part of the vast Orion Molecular Cloud Complex, the same star-forming region that contains the Orion Nebula (M42). M78 spans about 4 light-years across and shines at magnitude 8.3 by reflecting the light of two young B-type stars, HD 38563A and HD 38563B, embedded within its dusty clouds. Unlike emission nebulae which produce their own light, reflection nebulae shine by scattering starlight off microscopic dust grains, giving M78 its characteristic blue color in photographs. The region around M78 contains several other reflection nebulae (NGC 2064, NGC 2067, and NGC 2071) as well as dense dark nebulae.

Located about 2 degrees northeast of the star Alnitak (Zeta Orionis), the easternmost star of Orion's Belt. In binoculars, M78 appears as a small, hazy patch. A 4-inch telescope at 50-80x shows a comet-like smudge with two stars embedded in it. Larger apertures reveal the fan-shaped nebulosity and hints of the dark lanes that thread through the region. An OIII or UHC filter does not help with reflection nebulae — use no filter or a broadband light pollution filter instead. Best observed from November through February when Orion is prominent. The surrounding dark nebulae create an interesting contrast.

Discovered by Pierre Mechain in early 1780 and cataloged by Messier on December 17, 1780. Messier described it as 'two stars surrounded by nebulosity.' The nebula was one of the first objects to be studied by spectroscopy, confirming it shines by reflected starlight rather than emission. In 2004, the variable star V1647 Orionis erupted in the nearby McNeil's Nebula, briefly illuminating a patch of dust and creating a new reflection nebula that faded and reappeared multiple times.