Ring Nebula (M 57)

The Ring Nebula (M57), also known as M57, is a planetary nebula located in the constellation Lyra. This object was discovered by the French astronomer Antoine Dargage in 1779 and has since become one of the most famous objects in the sky.

The Ring Nebula gets its name from its shape, which resembles a ring or hole in the center. This is the result of the star at the center of the nebula, which was previously a red giant, emitting a shell of gas into outer space, creating this beautiful object.

The Ring Nebula is an object of interest to astronomers who study the processes of star formation and the evolution of stars. In addition, this object is popular among amateur astronomers due to its relative brightness and accessibility to observation even through a small telescope.

History

The nebula was discovered by the French astronomer Charles Messier while searching for comets in late January 1779. Messier's report of his independent discovery of Bode's Comet reached his colleague, the French astronomer Antoine Darquier de Pellepoix, twice later, who then independently rediscovered the nebula while following the comet. Darquier later reported that it was "as large as Jupiter and resembles a planet that is growing dim" (which may have contributed to the use of the term "planetary nebula").

In 1800, the German Count Friedrich von Hahn announced that he had discovered a faint central star at the center of the nebula several years earlier. He also noticed that the inner ring had undergone changes and said that he could no longer find the central star. In 1864, the English amateur astronomer William Huggins examined the spectra of several nebulae, finding that several of them, including M57, showed spectra of bright emission lines characteristic of fluorescent gases. Huggins concluded that most planetary nebulae were not composed of resolved stars, as had been previously thought, but were nebulae. The nebula was first photographed by the Hungarian astronomer Eugen von Gotthard in 1886.

Observations

M57 lies south of the bright star Vega, which forms the northwest corner of the Summer Triangle. The nebula lies about 40% of the way from Beta (β) to Gamma (γ) Lyrae, making it an easy target for amateur astronomers.

The nebula's disk has an angular size of 1.5 × 1 arcminute, making it too small to resolve with 10×50 binoculars. It is best seen with a telescope with an aperture of at least 20 cm (8 in), but even a 7.5 cm (3 in) telescope will reveal its elliptical ring shape. Using a UHC or OIII filter greatly improves visual observations, especially in light-polluted conditions. The inner hole can be resolved using a 10 cm (4 in) telescope at 100× magnification. Larger instruments will reveal several dark zones at the eastern and western edges of the ring and some faint elusiveness within the disk. The central star, with an apparent magnitude of 14.8, is difficult to see.

#Properties

M57 is 0.787 kpc (2,570 light-years) from Earth. It has an apparent magnitude of 8.8 and a photometric magnitude of 9.7. Photographs taken over 50 years show that the nebula is expanding at a rate of about 1 arcsecond per century, consistent with spectroscopic observations of 20–30 km/s. M57 is illuminated by a central white dwarf star with an apparent magnitude of 15.75.

The inner parts of the nebula have a bluish-greenish tint caused by emission lines of doubly ionized oxygen at 495.7 and 500.7 nm. These so-called "forbidden lines" are only visible at very low densities, containing a few atoms per cubic centimeter. In the outer part of the ring, some of the reddish tint is caused by emission of hydrogen at 656.3 nm, part of the Balmer line series. Forbidden lines of ionized nitrogen, or N II, contribute to the reddishness at 654.8 and 658.3 nm.

Nebula Structure

M57 belongs to a class of nebulae known as bipolar stellar nebulae, with thick equatorial rings that are clearly visible along the main axis of symmetry. It is a prelatal spheroid with strong concentrations of material along its equator. From Earth, the symmetry axis is viewed at an angle of about 30°. Overall, the observed emission from the nebula is currently estimated to be expanding for approximately 1610 ± 240 years.

Structural studies show that this planetary nebula has patterns characterized by well-developed symmetries. However, these are visible only as silhouettes against the background emission of the nebula's equatorial ring.

Central Star

The central star was discovered by the Hungarian astronomer Genő Gothard on September 1, 1886, in images taken at his observatory in Heren, near Szombatele. Over the last two thousand years, the central star of the Ring Nebula has left the asymptotic giant branch after exhausting its hydrogen fuel. It therefore no longer produces its energy through nuclear fusion and, in evolutionary terms, is now turning into a compact white dwarf star.

The central star is now composed mainly of carbon and oxygen with a thin outer shell of lighter elements. Its mass is about 0.61–0.62 M☉ and its surface temperature is 125,000±5,000 K. It is currently 200 times more luminous than the Sun, but its apparent magnitude is only +15.75.