M 76
M 76
M76 is among the fainter Messier objects. It is known under the names Little Dumbbell Nebula (the most common), Cork Nebula, and Butterfly 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" 193 H I.
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 diameter, probably material ejected as stellar winds from the central star when it was a Red Giant. 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 is not unusual for planetary nebulae, the distance is poorly known, with estimates between 1700 and 8200 (Kenneth Glyn Jones' value) light years. Accordingly, the true dimensions of the cork is between 0.34x0.72 and 1.7x3.5 light years, while the wings extend up to between 1.3 and 6.2 light years, and the faint halo reaches out to between 2.4 and 11.5 light years.
Bill Arnett's M76 photo page, info page.
| Right ascension | 01 : 38.8 (hours : minutes) |
|---|---|
| Declination | +51 : 19 (degrees : minutes) |
| Distance | 3.4 (light-years*10^3) |
| Visual magnitude | 10.2 |
