m57.gif M 57

Planetary Nebula M57 (NGC 6720), type 4+3, in Lyra

Ring Nebula

The famous ring nebula M57 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, as was first assumed by John Herschel. Viewed from this plane, it would thus more resemble the Dumbbell Nebula M27 than its current appearance 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.

Our color photo shows that the material of the Ring is exposing a decreasing ionization level with increasing distance from the 100,000 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 oxygene and nitrogene dominates the color, and in the outer region, only the red light of hydrogene can be excited.

As for most planetry 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.

Further images:

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 14.7-mag central star of the Ring. Tom Polakis has provided a webpage with photometric data of stars around M57.

Antoine Darquier de Pellepoix (Darquier), the discoverer of the Ring Nebula, 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. Curiously, 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 "none [of them] seems to belong to it."

Bill Arnett's Ring Nebula M57 photo page, info page.

Right Ascension 18 : 53.6 (hours : minutes)
Declination +33 : 02 (degrees : minutes)
Distance 4.1 (*1000 light-years)
Visual Magnitude 8.8
Apparent Dimension 1.4x1.0 (arc minutes)

Hartmut Frommert (spider@seds.org)

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