Messier 76, NGC 650, NGC 651 - Little Dumbbell Nebula

Messier 76 is a planetary nebula in the constellation Perseus, and one of only four planetary nebulae in Messier's catalog. It is also known as the Little Dumbbell Nebula, the Barbell Nebula, or the Cork Nebula. The Little Dumbbell's common name derives from its resemblance to the Dumbbell Nebula (M 27) in Vulpecula. Originally thought to consist of two separate nebulae, it was given two NGC catalog numbers, NGC 650 and 651.

Discovery and History

M 76 was discovered by Mechain in 1780, who reported it to Charles Messier, who observed it and added it to his catalog as number 76. While Mechain found it to be a nebula without stars, Messier thought it was composed of small stars with some nebulosity. Lord Rosse erroneously suspected some spiral structure in this nebula. In 1866, William Huggins, the pioneer of spectroscopy, found its spectrum to be gaseous.

William Herschel suspected M 76 be a "double nebula" with two components in contact, and numbered the second component H I.193. Pioneering astrophotographer Isaac Roberts found in 1891 that M 76 not double, but a single nebula. He suspected that it might be a broad ring, similar to the Ring Nebula (M 57), seen edgewise. In 1918, Heber Curtis correctly classified M 76 as a planetary nebula for the first time.

Amateur Observation

Located in the western part of Persues, M 76 is one of the faintest objects in Messier's list. With a visual magnitude of +10.1, it is surprising that Messier could see it with the telescope that he used. As is usual for planetary nebulae, M 76 is much brighter visually than photographically (mag 12.2). This is due to the fact that most of its visible light is emitted in the green, 5007 Angstrom "forbidden line" of doubly ionized oxygen.

Messier 76 does not have the usual circular disk shape of most planetary nebulae. Instead, it exhibits two bright nodules with a pinched center and irregular nebulous filaments coming out of both ends, surrounded by a much fainter, diffuse outer shell. The bright bar-shaped main body or 'cork' measures 42" x 87", with lower surface-brightness wings extending 157" x 87"; the faint halo covers a region about 290" in diameter.

NGC 651 is the north following (eastern) part of the nebula; NGC 650 is the south-western part. The central star has a visual magnitude of 15.9.

Properties and Structure

The distance to M 76 is not well known; estimates range from 1,700 to 15,000 light years. Consequently, its dimensions are also not well known. A current estimate of 3,400 light years makes the true dimensions of the cork 0.68 x 1.44 ly, while the wings extend up to 2.6 ly, and the faint halo reaches out to 4.8 ly. The whole planetary nebula is approaching us at 19.1 km/sec.

The bright bar-shaped main body is probably a slightly elliptical ring seen only a few degrees from edge-on. This ring seems to be expanding at about 42 km/s. Gas moving out more rapidly along the axis perpendicular to the ring plane forms the lower surface-brightness wings. Finally, the faint halo consists of material that was probably ejected from the central star in the form of stellar winds when it was still in its red giant phase.

Today, this central star has a high temperature of over 88,000 K, but will cool down as a white dwarf over the next few tens of billions of years.