M76 (planetary nebula in Perseus)
250: M76 Image
M76 was discovered by Pierre Méchain on September 5, 1780, and confirmed by Messier on October 21st. M76 is commonly called the Little Dumbbell Nebula, but is also known as the Cork Nebula, Butterfly Nebula, or the Barbell Nebula. Interestingly, although M76 is a single object, it has two NGC numbers because William Herschel and other early observers assumed that it was two separate objects. Visually, M76 looks like a tiny version of M27, the Dumbbell Nebula. The main body of the nebula is a bright elliptical ring seen nearly edge-on, with the expanding gas forming the lower surface brightness wings of the butterfly. These wings—and the 16.6 magnitude central star—are visible only in the largest amateur instruments. The brighter central portion is only about 42 by 87 arcseconds, which is to say about one arcminute in extent, which is only about 1/30th the diameter of the full moon.
The distance of M76 is uncertain, as is the case for most planetary nebulae. Estimates cover nearly a full order of magnitude, from 1,700 to 15,000 light years. As is common for planetary nebulae, M76 emits nearly all of its visible light in the 5007 O-III line, which means narrowband LPR filters and particularly O-III filters can be quite helpful in enhancing the contrast of the nebula against the background.
M76 is the dimmest object in the entire Messier list, and is by far the hardest object to locate of any those we’re talking about tonight. Not only is it dim, but it’s tiny, it’s not conveniently near any bright guidepost stars, and it’s not visible in standard binoculars or your optical finder. M76 lies almost exactly at the midpoint of a line drawn from Almaak in Andromeda to Ruchbah in Cassiopeia. That line is almost exactly 19 degrees long, with M76 lying about half-way along the line—9 degrees from Ruchbah and 10 degrees from Almaak.
260: M76 Telrad detail
The keys to locating M76 are to take the big-picture approach, use geometric relationships, and locate the relatively bright star 51 Andromedae (marked with the arrow). Once you have that star in your finder, pivot your scope 3 degrees—about half a finder field—on the line that links 51 Andromedae to 45 Epsilon Cassiopeia. M76 lies just off that line, where it will appear in your low-power eyepiece as a slightly fuzzy star. Once you have that fuzzy star centered, change to a high-power eyepiece to view the object.