Messier 57

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, thus coinciding with an early assumption by John Herschel. Viewed from this equatorial plane, it would thus more resemble the Dumbbell Nebula M27 or the Little Dumbbell Nebula M76 than its appearance we know from here: We happen to view it from near one pole.

This is contrary to the belief expressed e.g. in Kenneth Glyn Jones' book. There are even indications from investigations of deep observations such as George Jacoby's deep photos obtained at Kitt Peak National Observatory that the overall shape might be more that of a cylinder viewed along the direction of the axis than that of a ring, i.e., we are looking down a tunnel of gas ejected by a star at the end of its nuclear-burning life. Eventually, these observations have given evidence that the equatorial ring or cylinder has lobe-shaped extensions in polar directions, similar to those found in deep images of M76, but even more resembling other planetaries like NGC 6302, see e.g. the review by Sun Kwok (2000).

The 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 by J.C. Duncan (Duncan, 1935).

Our color photo (taken with the 200-inch Hale telescope at Mt. Palomar) shows that the material of the Ring is exposing a decreasing ionization level with increasing distance from the 100,000 to 120,000 K hot central star. The innermost region appears dark as it emits merely UV radiation, while in the inner visible ring, greenish forbidden light of ionized oxygen and nitrogen dominates the color, and in the outer region, only the red light of hydrogen can be excited.

The central star was discovered in 1800 by the German astronomer Friedrich von Hahn (1742-1805), with a 20-foot FL reflector. This object is a planet-sized white dwarf star, which shines at about 15th magnitude. It is the remainder of a sunlike star, probably once of more mass than our sun, which has blown away its outer envelopes at the end of its Mira-like phase of evolution. Now over 100,000 K hot, it will soon start to cool down, shine as a white dwarf star for a while of several billions of years, and then eventually end as a cold Black Dwarf.

As for most planetary nebulae, the distance to the Ring Nebula M57 is not very wellknown. In case of this nebula, however, attempt was made to relate its angular expansion rate of roughly 1 arc second per century with its radial expansion velocity. These results, however, were based on wrong assumptions of the geometry of this nebula, presuming a spherical shape. Therefore, until recently, only rough estimates could be made, based on various theoretical assumptions and models. The following distance values have been given: 4,100 ly (K.M. Cudworth 1974; Mallas/Kreimer), 1,410 ly (Kenneth Glyn Jones), 2,000 to 2,500 ly (Vehrenberg), 2,000 ly (Sky Catalogue 2000.0), "more than 2,000 ly" (Murdin/Allen's Catalogue of the Universe), 5,000 ly (Chartand/Wimmer's Skyguide), 3,000 ly (WIYN), and 1,000 to 2,000 ly (Sun Kwok, 2000). A good value for the distance still needs to be determined (e.g., parallax by Hubble Space Telescope), but recently improved CCD technics was 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 under the assumption of constant expansion. For its extension of 60x80 arc seconds, this yields a time of expansion of about 6,000 to 8,000 years.

As most planetary nebulae, the Ring is much brighter visually at magnitude 8.8 than photographically at only 9.7 mag; a consequence of the fact that most light is emitted in very few particular spectral lines (see the discussion in our planetary nebulae page). Assuming a distance of 2,300 lightyears, this corresponds to an absolute magnitude of -0.3 visually (+0.5 photographically), or an intrinsic brightness of about 50 to 100 times that of our Sun. Even the 14.7-mag central star, of the size of a terrestrial planet, is only little fainter than our Sun with an absolute magnitude of about +5 or 6. Its apparent dimension of 1.4 arc minutes corresponds to a linear diameter of 0.9 lightyears (5.5 trillion miles or 8.8 trillion km, or 60,000 Astronomical Units), the halo extending out to a diameter of 2.4 lightyears.

The mass of the nebular matter has been estimated at about 0.2 solar masses, the density at about 10,000 ions per ccm (cm^3). Its chemical composition has been determined as follows: On each Fluor (Fl) atom, the Ring Nebula contains 4.25 million atoms of Hydrogen (H), 337,500 Helium (He), 2,500 Oxygen (O), 1,250 Nitrogen (N), 375 Neon (Ne), 225 Sulfur (S), 30 Argon (Ar) and 9 Chlorine (Cl) atoms. It is expanding at 20 to 30 km/s, and approaching us at 21 km/s.

M57 images by Finnish astronomers have shown a star which is superimposed (before or behind) over the ring.

For amateurs, it is always a challenge to identify the faint central star of the Ring. Note Tom Polakis' photometric data of stars around M57 and the Photometry of M57 Field Stars, by Brian Skiff.

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.

Of the neighboring stars, Beta Lyrae (Sheliak) is a notable eclipsing binary, with components of spectral type B7 and A8, varying between mag 3.4 and 4.4 with a period of 12.91 days. Gamma Lyrae (Sulaphat, Arabic for "Tortoise") is a giant of spectral type B9 III and mag 3.2 with a mag 12 companion at 13.8" distance in position angle 300 deg. The 0.4' small and 14.4-mag faint galaxy IC 1296 is situated just 4' NW of M57 and can be found with large instruments.

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