Messier 57, NGC 6720 - Ring Nebula

Messier 57 (NGC 6720) is the famous Ring Nebula. It is a showpiece in the northern hemisphere summer sky, and often regarded as the prototype of all planetary nebulae. These objects are the remains of sunlike stars which have blown away their outer envelopes, leaving planet-sized white dwarfs at their centers.

Discovery and History

The Ring Nebula was discovered by the French Astronomer Antoine Darquier de Pellepoix in January, 1779. He described it as "a dull nebula, but perfectly outlined; as large as Jupiter and looks like a fading planet." Only a few days later, Charles Messier independently found the same nebula while searching for comets, and entered it as the 57th object in his catalogue. Messier, and later William Herschel, speculated that the nebula was formed by multiple faint stars that they were unable to resolve.

William Herschel, who discovered the planet Uranus, found other nebulous objects resembling M 57 (and his newly discovered planet), and introduced the term "Planetary Nebulae" for them. Yet, oddly, Herschel did not count their most prominent representative among them. He instead considered M 57 to be a peculiar object, a "curiosity of the heavens", and described it as "a perforated nebula, or ring of stars"; this was the first mention of its ring shape. Herschel also identified some of the superimposed stars, and correctly assumed that "none [of them] seems to belong to it."

In 1800, Count Friedrich von Hahn discovered the faint central star at the heart of the Ring Nebula. In 1864, William Huggins examined the spectrum of M 57, finding that it displayed the bright emission lines characteristic of fluorescing gases. Huggins concluded that most planetary nebulae were not composed of unresolved stars, as had been previously suspected, but of glowing gas.

Appearance and Observation

M 57 is easy to locate, about 40% of the distance from Beta to Gamma Lyrae (the two lower stars in the parallelogram of Lyra). As with most planetary nebulae, the Ring is much brighter visually (magnitude 8.8) than photographically (mag 9.7). It can be seen with binoculars as an almost stellar object, but is difficult to identify because of its small apparent diameter (1.4' x 1.0').

Although a 3-inch telescope will show the ring, M 57 is best seen through at least an 8-inch telescope. It is a truly magnificent object in instruments of moderate aperture: a bright oval smoke ring, the center filled with a faint ghostly haze. The ring becomes apparent at about 100x magnification, with a 12th-magnitude star about 1' east of the center. A halo of very faint material extends off to over 3.5'.

The slight ellipticity of the Ring Nebula is apparent even in small scopes. With increasing aperture, and under good conditions, more detail becomes visible, and the Ring appears slightly greenish in color. At high power, some subtle variation in surface brightness may be glimpsed, and the edges of the Ring's major axis appear ragged, with wisps of faint nebulosity. The very faint, magnitude 15.75 central star is visible only under ideal conditions in large telescopes, or with the help of filters. It appears along side several very faint foreground or background stars, and identifying it is a challenge for amateurs.

Properties and Structure

As with most planetary nebulae, the distance to the Ring Nebula is not very well known. Photographically, the nebula's rate of expansion is roughly 1" per century, while its radial expansion velocity (from spectroscopic observations) is 20 to 30 km/sec. Attempts to relate its angular expansion rate to the radial expansion give rough distance estimates from 1,410 to 4,100 light years. These results, however, presume various theoretical models, including a spherical geometry for the nebula, an assumption that is probably wrong. The US Naval Observatory and Hubble Space Telescope used improved techniques in 1997-99 to determine a trigonometric parallax for M 57's the central star, yielding a distance of 2,300 light years (with 40% uncertainty!)

Assuming this distance, the Ring's apparent dimensions correspond to a linear diameter of 0.9 x 0.7 light years; its halo extends out to 2.4 light years. Its visual magnitude of 8.8 corresponds to an absolute magnitude of -0.3, or an intrinsic brightness about 100 times the Sun's. Assuming constant expansion, the age of the Ring Nebula can be roughly estimated at 6,000 to 8,000 years. It is approaching us at 21 km/sec.

Recent research has confirmed that M 57 is actually a torus of light-emitting material, not a spherical or ellipsoidal shell. Its overall shape might even be that of a cylinder or hourglass, which we happen to view at an angle of about 30° from one pole. In other words, we are looking down the mouth of a tunnel of gas whose symmetry axis is pointing toward the Earth, giving it a ring-like appearance.

The Ring Nebula has been estimated to contain about 0.2 solar masses, with a density of about 10,000 ions per cm3. The innermost region appears darker because it emits mainly UV radiation, and has a blue-green tinge emitted by doubly-ionized oxygen. The reddish hue in the outer region is caused by emission from ionized hydrogen and nitrogen.

Evolution and Central Star

Planetary nebulae are formed after medium- or low-mass stars, like the Sun, exhaust the hydrogen fuel in their cores. At this point, the stars' outer layers expand, and they becomes red giants. Further internal instabilities cause their outer atmospheres to be expelled in energetic pulses. The expanding gaseous shell forms the planetary nebula, while the stellar core collapses to become a white dwarf, brightly illuminating the nebula with ultraviolet energy from its super-hot surface.

The central star illuminating M 57 contains approximately 1.2 solar masses. This 15th-magnitude white dwarf, now the size of a terrestrial planet, is actually fainter than our Sun, with an absolute magnitude of +5 to +6. Although it currently has a surface temperature of 100,000 to 120,000 K, it will cool over several billion years, and eventually end up as a cold, dead, "black dwarf".