Messier 76

Dicovered by Pierre Méchain in 1780.

Planetary Nebula Messier 76 (M76, NGC 650/651) is one of the faintest Messier Objects, and one of only four planetary nebulae in Messier's catalog, situated in the Eastern part of constellation Perseus.

M76 was discovered by Pierre Méchain on September 5, 1780, who reported it to Charles Messier, who observed it on October 21, 1780, determined its position and added it to his catalog. While Méchain found it to be a nebula without stars, Messier thought it was composed of small stars with some nebulosity, probably being fooled by foreground or background stars. Lord Rosse erroneously suspected to have detected some spiral structure in this nebula. In 1866, William Huggins, the pioneer of spectroscopy, found its spectrum to be gaseous, showing Nebulium lines. Pioneer astrophotographer Isaac Roberts found that this was not a double, but a single nebula, and first suspected it might be a broad ring seen edgewise. In 1918, Heber D. Curtis correctly classified it as a planetary nebula for the first time.

M76 is among the fainter Messier objects. It is known under the names Little Dumbbell Nebula (the most common), Cork Nebula, Butterfly Nebula, and Barbell Nebula, and it was given two NGC numbers as it was suspected to be a double nebula with two components in contact, a hypothesis brought up by William Herschel, who numbered the "second component" H I.193 on November 12, 1787. NGC 651 is the North following (East) part of the nebula.

The appearance of M76 resembles to some degree that of the Dumbbell Nebula M27. Most probably, the main body (the bar, or cork) is a bright and slightly elliptical ring we see edge-on, from only a few degrees off its equatorial plane. This ring seems to expand at about 42 km/sec. Along the axis perpendicular to this plane, the gas expands significantly more rapidly to form the lower surface brightness "wings" of the butterfly.

While the bright part of the nebula is of about 65 arc seconds in diameter (more accurately, the `cork' is about 42x87", the `wings' 157x87"), this nebula is surrounded by a faint halo covering a region of 290 arc seconds in diameter (Millikan, 1974); this material was probably ejected in the form of stellar winds from the central star when it was still in the Red Giant phase of evolution. Today the central star is of mag 16.6 and a high temperature of some 60,000 K, which will probably cool down as a white dwarf over the coming tens of billions of years.

As usual for planetary nebulae, M76's visual magnitude is much brighter (9.6 according to Don Machholz' personal estimate, 10.1 according to Hynes; the present author thinks this is close to his own perception) than photographically (most sources agree on 12.2 mag photographically). This is due to the fact that most visual light is emitted in one spectral line, the green 5007 Angstrom forbidden line of doubly ionized oxygen, [O III] (see our Planetary Nebulae page).

As is not unusual for planetary nebulae, the distance is poorly known, with estimates between 1,700 and 15,000 light years (the latter value is from Kaufmann's Universe; Kenneth Glyn Jones has the value of 8,200). Accordingly, the true dimensions of the cork is between 0.34x0.72 and 3.1x6.4 light years, while the wings extend up to between 1.3 and 11.3 light years, and the faint halo reaches out to between 2.4 and 21 light years. (Our 3400 light years yield 0.68x1.44, 2.6, and 4.8 light years, while with Kenneth Glyn Jones' distance, the cork is 1.7x3.5, the wings 6.2, and the extensions 11.5 light years).

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