M78 is a fine example of a reflection nebula. The ultraviolet radiation from the stars that illuminate it is not intense enough to ionize the gas to make it glow — its dust particles simply reflect the starlight that falls on them. Despite this, M78 is easily observable through a small telescope because it is one of the brightest reflection nebulae in the sky. It lies about 1,350 light-years away in the constellation Orion The Hunter, and it can be found northeast of the easternmost star of Orion’s Belt.
The pale blue tint seen in the nebula in this picture is an accurate representation of its dominant color. Blue hues are commonly seen in reflection nebulae because of the way the starlight is scattered by the tiny dust particles they contain — the shorter wavelength of blue light is scattered more efficiently than the longer wavelength of red light.
This image contains many other striking features apart from the glowing nebula. A thick band of obscuring dust stretches across the image from the upper left to the lower right, blocking the light from background stars. In the bottom right corner, many curious pink structures are also visible, which are created by jets of material being ejected from stars that have recently formed and are still buried deep in dust clouds.
Two bright stars, HD 38563A and HD 38563B, are the main powerhouses behind M78. However, the nebula is home to many more stars, including a collection of about 45 low-mass young stars — less than 10 million years old — in which the cores are still too cool for hydrogen fusion to start, known as T Tauri stars. Studying T Tauri stars is important for understanding the early stages of star formation and how planetary systems are created.
Remarkably, this complex of nebulae has also changed significantly in the past 10 years. In February 2004, the experienced amateur observer Jay McNeil took an image of this region with a 75mm telescope and was surprised to see a bright nebula — the prominent fan-shaped feature near the bottom of this picture — where nothing was seen on most earlier images. This object is now known as McNeil’s Nebula, and it appears to be a highly variable reflection nebula around a young star.
This color picture was created from many monochrome exposures taken through blue, yellow/green, and red filters, supplemented by exposures through an Hydrogen-alpha filter that shows light from glowing hydrogen gas. The total exposure times were 9, 9, 17.5, and 15.5 minutes per filter, respectively.
