M27 - The Dumbbell Nebula in Vulpecula
Discovered by Charles Messier in 1764.
The Dumbbell Nebula M27 was the first planetary nebula ever discovered. On July 12, 1764, Charles Messier discovered this new and fascinating class of objects, and describes this one as an oval nebula without stars. The name "Dumb-bell" goes back to the description by John Herschel, who also compared it to a "double-headed shot."
We happen to see this one approximately from its equatorial plane (approx. left-to-right in our image); this is similar to our view of another, fainter Messier planetary nebula, M76, which is called the Little Dumbbell. From near one pole, it would probably have the shape of a ring, and perhaps look like we view the Ring Nebula M57.
This planetary nebula is certainly the most impressive object of its kind in the sky, as the angular diameter of the luminous body is nearly 6 arc minutes, with a faint halo extensing out to over 15', half the apparent diameter of the Moon (Millikan 1974). It is also among the brightest.
The bright portion of the nebula is apparently expanding at a rate of 6.8 arc seconds per century, leading to an estimated age of 3,000 to 4,000 years, i.e. the shell ejection probably would have been observable this time ago (it actually happened earlier as the light had to travel all the distance of perhaps about 1000 light years).
The central star of M27 is quite bright at mag 13.5, and an extremely hot blueish subdwarf dwarf at about 85,000 K (so the spectral type is given as O7 in the Sky Catalog 2000). K.M. Cudworth of the Yerkes Observatory found that it probably has a faint (mag 17) yellow companion.
As for most planetary nebulae, the distance of M27 (and thus true dimension and intrinsic luminosity) is not very well known. Hynes gives about 800, Kenneth Glyn Jones 975, Mallas/Kreimer 1250 light years, while other existing estimates reach from 490 to 3500 light years. Currently, investigations with the Hubble Space Telescope are under way to determine a more reliable and acurate value for the distance of the Dumbbell Nebula.-------------------------------