Messier 78, or NGC 2068, is the brightest reflection nebula in the sky, located in Orion near the celestial equator.
This object was discovered by Pierre Mechain in early 1780; Charles Messier added it to his catalog late that year.
Observing Messier 78
M 78 is easy to locate, about 3° northeast of Zeta Orionis (Alnitak), the easternmost star in Orion's Belt. Visually, M 78 resembles a faint comet. It is just visible in binoculars under good conditions, as a very dim nebulous patch about 8' x 6' in size. Small telescopes show it remarkably well, and reveal the two illuminating stars, which appear like a double nucleus in the compact "comet head". These two stars, HD 38563A and HD 38563B, are responsible for making the cloud of dust in M 78 visible. The nebula fans out to the southwest, and suggests a faint comet tail.
The other nebulae in this field require a very dark sky, and are much more difficult to see. Under very good conditions, a 4-inch scope can reveal NGC 2071, and suggestions of haze around M 78. There are fewer stars to the west, an indication that dark nebulae in this region obscure the stellar background.
About 1-3/4° east of M 78, the open cluster NGC 2112 is found; this cluster is of about 9th magnitude and 11' in diameter. It lies behind M 78 at a distance of about 2800 light-years, and is much older, estimated at about 2 billion years.
Properties and Evolution
As a reflection nebula, M 78 is a cloud of interstellar dust which shines by the reflected and scattered light of nearby stars. Its nature as a reflection nebula was discovered by Vesto Slipher of Lowell Observatory in 1919. M 78 exhibits a continuous spectrum, which exactly mirrors that of the bright stars illuminating it, rather than the discrete-line spectra of emission nebulae (like M 42) which glow by fluorescence.
M 78 is part of the Orion complex, a large cloud of gas and dust centered on the Orion Nebula (M 42), and is about 1,600 light years distant. It is the brightest portion of a vast dust cloud which includes NGC 2024 (the Flame Nebula) near Zeta Orionis, NGC 2071, NGC 2067, and the very faint NGC 2064. All these nebulae are associated with the molecular cloud LDN 1630.
About 45 variable stars of the T Tauri type are known in M 78. Stars of this type are young main sequence stars of spectral type F or G, similar to our sun, still in the process of formation. They are 4 to 5 times brighter than their spectral type would suggest, are associated with bright or dark nebulosity, and vary in brightness by about 3 magnitudes.
Infrared investigations have found a cluster of 192 young stars, spread over an area of 7' in diameter, which have formed in this nebula. Messier 78 also includes some 17 Herbig-Haro objects, which are jets of matter ejected from young stars, embedded in the nebulosity where they have just formed.