The Ring Nebula in Lyra
Discovered by Antoine Darquier de Pellepoix in 1779.
The famous ring nebula Messier 57 (M57, NGC 6720) is often regarded as the prototype of a planetary nebula, and a showpiece in the northern hemisphere summer sky.
Recent research has confirmed that it is, most probably, actually a ring (torus) of bright light-emitting material surrounding its central star, and not a spherical (or ellipsoidal) shell. We happen to view it from near one pole.
Deep observations also show an extended halo of material extending off to over 3.5 arc minutes (Hynes gives 216 arc seconds, quoting Moreno & Lopez, 1987), remainders of the star's earlier stellar winds. The halo was discovered in 1935.
A good value for the distance still needs to be determined (e.g., parallax by Hubble Space Telescope), but recently improved CCD technics were used at the US Naval Observatory (USNO) to determine a trigonometric parallax for the central star of M57, yielding 2,300 ly (Harris et.al. 1997, see also STScI/Nasa, Jan 1999).
From the expansion rate of one arc second per century given above, the age of the nebula can be roughly estimated as about 6,000 to 8,000 years.
For amateurs, it is always a challenge to identify the faint central star of the Ring.
M57 was the second planetary nebula to be discovered (in January 1779), 15 years after the first one, M27. Antoine Darquier de Pellepoix (Darquier), who discovered the Ring Nebula only a few days before Charles Messier found and cataloged it, described it as "a dull nebula, but perfectly outlined; as large as Jupiter and looks like a fading planet." This comparison to a planet may have influenced William Herschel, who found the objects of this type resembling the planet newly discovered by him, Uranus, and introduced the name "Planetary Nebulae". Herschel described M57 as "a perforated nebula, or ring of stars;" this was the first mention of the ring shape. Oddly, the inventor of the name "Planetary Nebula" did not count this most prominent representative in this object class, but described it as a "curiosity of the heavens", a peculiar object. Herschel also identified some of the superimposed stars, and correctly assumed that "none [of them] seems to belong to it."
M57 is very easy to locate as it is situated between Beta and Gamma Lyrae, at about one-third the distance from Beta to Gamma. It can be seen with binoculars as an almost stellar object, difficult to identify just because of its small apparent diameter. In smaller amateur telescopes, the ring becomes apparent at about 100 magnification, with a darker middle; a 12th-mag star is east of the planetary nebula, about 1' of the center. If ever color is notable, the Ring Nebula appears slightly greenish, not unexpected because most of its light is emitted in few green spectral lines. Even in small scopes, a slight ellipticity can be noted, with major axis in a position angle of about 60 deg. With increasing aperture and under good condition, more and more detail becomes visible, but even in large instruments, the central star will be apparent only under exceptionally good conditions, or with the help of filters. In large instruments, several very faint foreground or background stars can be glimpsed within the nebula's extension under very good conditions.
-----------------------------------------------------------------Scope: Planewave 12.5" CDK