M 27, commonly known as the Dumbbell Nebula, is the first planetary nebula ever discovered, and perhaps the finest in the sky. It is easily seen in binoculars, and a popular observing target for amateur telescopes.
Discovery and Observation
Charles Messier discovered this object in 1764, and described it as an oval nebula without stars. William Herschel invented the name "Planetary Nebula" for this class of objects around 1784, because he found them to resemble his newly discovered planet, Uranus. The name "Dumbbell" goes back to William's son John Herschel, who compared M 27 to a "double-headed shot".
This planetary nebula is certainly the most impressive object of its kind in the sky. But at visual magnitude 7.4, it only the second brightest. While the Helix Nebula in Aquarius (NGC 7293) is slightly brighter, at magnitude 7.3, it a much lower surface brightness because of its larger size.
The two bright triangular lobes that give rise to the Dumbbell's name extend north and south to an overall length of 8' x 6'. A faint halo extends out to over 15', half the apparent diameter of the Moon. Several stars are superimposed on the nebula, the most conspicuous in the center of the northern lobe.
Like many nearby planetary nebulae, the Dumbbell contains knots. Its central region is marked by a pattern of dark and bright cusped knots and their associated dark tails. The knots vary in appearance from symmetric objects with tails to rather irregular tail-less objects.
The central star of M 27, just visible at magnitude 13.5, is surrounded by a slightly darker area. It has a faint (mag 17) yellow companion 6.5" away. About 2 degrees to the west of M 27 is inconspicuous open cluster, NGC 6830, which contains about 20 - 30 widely scattered stars; this cluster is about 5500 light years distant.
Properties and Evolution
As with most planetary nebulae, the distance to M 27 is not very well known. Existing estimates range from 490 to 3500 light years. Adopting a value of 1200 light years, the nebula's intrinsic luminosity is about 100 times the Sun's. Its central star has about 1/3 of the Sun's luminosity, and its companion is nearly 100 times fainter.
As with all planetary nebulae, M 27's central white dwarf is the collapsed core of a formerly sunlike star which, after exhausting its hydrogen fuel, become a red giant and ejected its outer layers. The expanding gas shell is excited by high-energy radiation from the central white dwarf star, forming the visible nebula. The star is so much fainter than the nebula because it emits mainly in the non-visible part of the spectrum.
The central white dwarf of M 27 is contains an estimated 60% of the Sun's mass. It is about 5% of the Sun's diameter, making it the largest white dwarf known. With a surface temperature of 85,000 K, this blue-white star has a spectral type of O7. The high-energy ultraviolet radiation from its extremely hot surface is absorbed by the nebula's gas, and re-emitted as visible light. Most of this light is emitted at only one wavelength, 500.7 nm, which corresponds to the greenish spectral line of doubly-ionized oxygen.
We happen to see M 27 approximately from its equatorial plane - similar to our view of another, fainter Messier planetary nebula, M 76 in Perseus, also called the "Little Dumbbell". Viewed from its poles, M 27 would probably appear ring-shaped, like M 57, the Ring Nebula in Lyra. The knots in the Dumbbell Nebula have have bright cusps which are local photoionization fronts, similar to those in the Helix and Eskimo Nebulae.
Like its distance, the Dumbbell Nebula's age is also uncertain. By one estimate, the bright portion of the nebula is expanding at a rate of 6.8" per century, leading to an age of 3,000 to 4,000 years. By another, the expansion rate is no more than 2.3" per century, giving an upper age limit of about 15,000 years. Spectroscopic observations indicate that the nebula has an expansion velocity of 31 km/sec. With a physical radius of about 1 light year, the kinematic age of the planetary nebula is approximately 9,800 years.