Ring Nebula

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The Ring Nebula

M57, The Ring Nebula. Image credit: NASA/ESA
Observation data
(Epoch J2000)
Right ascension 18h 53m 35.079s[1]
Declination +33° 01′ 45.03″[1]
Distance 2.3+1.5−0.7 kly (700+450−200 pc)[2]
Apparent magnitude (V) 9[3]
Apparent dimensions (V) 230″ × 230″[2]
Constellation Lyra
Physical characteristics
Radius 1.3+0.8−0.4 ly[a]
Absolute magnitude (V) -0.2+0.7−1.8[b]
Notable features -
Other designations M 57,[1] NGC 6720[1]
See also: Planetary nebula, Lists of nebulae

The Ring Nebula (also known as the Messier 57 or NGC 6720) is located in the constellation Lyra. It is among the most well known and recognizable examples of a planetary nebula.

Contents

[edit] Observation

Location of M57 in the constellation Lyra.
Location of M57 in the constellation Lyra.

M57 is located in Lyra, south of its brightest star Vega. Vega is the northeastern vertex of the three stars of the Summer Triangle. M57 lies about 40% of the angular distance from β Lyrae to γ Lyrae.[4]

M57 is best seen through at least an 8-inch telescope, but even a 3-inch telescope will show the ring.[4] Larger instruments will show a few darker zones on the eastern and western edges of the ring, and some faint nebulosity inside the disk.

This nebula was discovered by Antoine Darquier de Pellepoix in January, 1779. He reported that it was "as large as Jupiter & resembles a planet which is fading." Later the same month, Charles Messier independently made the same discovery while searching for a comet. It was entered into his list as the 57th object, giving it the designation M57. He speculated that the nebula was formed of multiple faint stars that he was unable to resolve with his telescope.[5][6]

In 1800, Count Friedrich von Hahn discovered a faint central star at the center of the nebula. In 1864, William Huggins examined the spectra of multiple nebulae. He discovered that some of these objects, including M57, displayed a spectrum of bright emission lines; a characteristic of a glowing gas. He concluded that these nebulae were not composed of unresolved stars, as had been previously suspected.[7][8]

[edit] Properties

The Ring Nebula in infrared.
The Ring Nebula in infrared.

The nebula is located at 0.7 kpc (2300 light-years) from Earth. The nebula has a visual magnitude of 8.8, and a photographic magnitude of 9.7. It is expanding at a rate of approximately 1 arcsecond per century (corresponding to 20–30 km/s), and this growth has been observed directly from photographs taken at intervals of 50 years.[9] Its mass is approximately 1.2 solar mass. M57 is illuminated by a central white dwarf of 15.75[10] visual magnitude.

The interior parts of this nebula have a green tinge that is caused by the emission lines at 495.7 and 500.7 nm from doubly-ionized oxygen. These so-called "forbidden lines" are observed only in conditions of very low density, with a few atoms per cubic centimeter. In the outer region of the ring, part of the reddish hue is caused by hydrogen emission at 656.3 nm, which forms part of the Balmer series of lines. Ionized nitrogen also contribues to the red with forbidden lines at 654.8 and 658.3 nm.[9]

[edit] Structure

M 57 is estimated to have been expanding for approximately 1,610 ± 240 years. It is bipolar, that is, it has thick equatorial rings with extended structure along its axis of symmetry. It appears to be a prolate spheroid with strong concentrations of material in its equator. Such a structure is a natural product of a bipolar model. From earth, it is viewed at about 30° from the symmetry axis.

M 57 exhibits knots characterized by a developed sense of symmetry. However, they are only visible as a silhouette against the background emission from the nebula's equatorial ring. M 57 may include N II emission located at the tips of the knots facing the central star. However, most of the knots are neutral and appear only in extinction. The existence of some knots with possible N II emission shows that they are located closer to the ionization front than those found in IC 4406. Some of the knots exhibit well developed tails which are often of a detectable optical thickness in the visual spectrum.[11][2]

[edit] Central star

A planetary nebula is formed after a medium or low mass star, such as the Sun, has exhausted the hydrogen fuel at its core. At this point the star changes form to achieve a new equilibrium condition in which it can burn helium fuel. By so doing, the outer layers expand and the star becomes a red giant. After about a billion years, temperature instabilities develop from the fusion reactions inside the star, causing the outer atmosphere to be ejected in a series of energetic pulses. This expanding gaseous shell forms a spherical nebula that is illuminated by ultraviolet energy from the star at its center.[12]

Within the past two thousand years, the star at the center of the nebula left the asymptotic giant branch after exhausting its supply of hydrogen fuel. This means it is no longer producing energy through nuclear fusion. It is now approaching the final stage of its evolutionary life where it begins to steadily cool down as a compact white dwarf. This star now consists primarily of carbon and oxygen, with a thin outer envelope of lighter elements. It has about 0.61–0.62 solar masses and a surface temperature of 125,000±5,000 K. Though currently 200 times as luminous as the Sun, it has a visual magnitude of only 15.75.[10]

[edit] See also

[edit] Notes

  1. ^ Radius = distance × sin(angular size / 2) = 2.3+1.5−0.7 kly * sin(230″ / 2) = 1.3+0.8−0.4 ly
  2. ^ 9 apparent magnitude - 5 * (log10(700+450−200 pc distance) - 1) = -0.2+0.7−1.8 absolute magnitude

[edit] References

  1. ^ a b c d SIMBAD Astronomical Database. Results for Messier 57. Retrieved on 2006-12-19.
  2. ^ a b c O'Dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; Burkert, A. (2002). "Knots in Nearby Planetary Nebulae". The Astronomical Journal 123 (6): 3329-3347. doi:10.1086/340726. 
  3. ^ Murdin, P. (2000). "Ring Nebula (M57, NGC 6720)". Encyclopedia of Astronomy and Astrophysics, Edited by Paul Murdin, article 5323. Bristol: Institute of Physics Publishing, 2001. http://eaa.iop.org/abstract/0333750888/5323. 
  4. ^ a b Crossen, Craig; Rhemann, Gerald (2004). Sky Vistas: Astronomy for Binoculars and Richest-Field Telescopes. Springer. ISBN 3211008519. 
  5. ^ Garfinkle, Robert A. (1997). Star-hopping: Your Visa to Viewing the Universe. Cambridge University Press. ISBN 0521598893. 
  6. ^ Messier, Charles (1780). Catalogue des Nébuleuses & des amas d'Étoiles, Connoissance des Temps for 1783, 225–249. 
  7. ^ Frommert, Hartmut; Kronberg, Christine. William Huggins (February 7, 1824 - May 12, 1910). Students for the Exploration and Development of Space. Retrieved on 2008-04-11.
  8. ^ Huggins, W.; Miller, W. A. (1863–1864). "On the Spectra of Some of the Nebulae. And On the Spectra of Some of the Fixed Stars.". Proceedings of the Royal Society of London 13: 491–493. 
  9. ^ a b Karttunen, Hannu (2003). Fundamental Astronomy. Springer, p. 314. ISBN 3540001794. 
  10. ^ a b O'Dell, C. R.; Sabbadin, F.; Henney, W. J. (2007). "The Three-Dimensional Ionization Structure and Evolution of NGC 6720, The Ring Nebula". The Astronomical Journal 134 (4): 1679–1692. 
  11. ^ O'dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; Burkert, A. (2003). "Knots in Planetary Nebulae". Winds, Bubbles, and Explosions: a conference to honor John Dyson, Pátzcuaro, Michoacán, México, September 9-13, 2002 (Eds. S. J. Arthur & W. J. Henney) Revista Mexicana de Astronomía y Astrofísica (Serie de Conferencias) (http://www.astroscu.unam.mx/~rmaa/) 15: 29-33. 
  12. ^ De Loore, C. W. H.; Doom, C. (1992). Structure and Evolution of Single and Binary Stars. Springer. ISBN 0792317688. 

[edit] External links