Helix Nebula

The Helix Nebula (also known as NGC 7293 or Caldwell 63) is a planetary nebula (PN) located in the constellation Aquarius. Discovered by Karl Ludwig Harding, probably before 1824, this object is one of the closest to the Earth of all the bright planetary nebulae.[4] The distance, measured by the Gaia mission, is 655±13 light-years.[5] It is similar in appearance to the Cat's Eye Nebula and the Ring Nebula, whose size, age, and physical characteristics are similar to the Dumbbell Nebula, varying only in its relative proximity and the appearance from the equatorial viewing angle.[6] The Helix Nebula has sometimes been referred to as the "Eye of God" in pop culture,[7] as well as the "Eye of Sauron".[8][9]

For the different but similarly named nebula, see Double Helix Nebula.
"Eye of God" redirects here. For other uses, see Eye of God (disambiguation).
Helix Nebula, NGC 7293
Emission nebula
Planetary nebula
(Infrared image version) Helix Nebula taken by the Spitzer Space Telescope (Infrared, 2007[1])
Credit: NASA, JPL, Caltech, Kate Su (Steward Obs., U. Arizona).
Observation data: J2000 epoch
Right ascension22h 29m 38.55s[2]
Declination−20° 50′ 13.6″[2]
Distance650 ly (200+1
−1 pc)(Gaia) ly
Apparent magnitude (V)+7.6[2]
Apparent dimensions (V)25′[3]
ConstellationAquarius
Physical characteristics
Radius2.87 ly (0.88 pc)[3] ly
Notable featuresOne of the nearest PNe
DesignationsNGC 7293[2] Caldwell 63

General information

The Helix Nebula is an example of a planetary nebula, formed by an intermediate to low-mass star, which sheds its outer layers near the end of its evolution. Gases from the star in the surrounding space appear, from our vantage point, as if we are looking down a helix structure. The remnant central stellar core, known as the central star (CS) of the planetary nebula, is destined to become a white dwarf star. The observed glow of the central star is so energetic that it causes the previously expelled gases to brightly fluoresce.

The nebula is in the constellation of Aquarius, and lies about 650 light-years away, spanning about 0.8 parsecs (2.5 light-years). Its age is estimated to be 10600+2300
−1200 years, based on its measured expansion rate of 31 km·s−1.[6]

Structure
A 3 dimensional map of carbon monoxide in NGC 7293 [10]
NGC 7293 seen through several visible filters by Hubble Space Telescope

The Helix Nebula is thought to be shaped like a prolate spheroid with strong density concentrations toward the filled disk along the equatorial plane, whose major axis is inclined about 21° to 37° from our vantage point. The size of the inner disk is 8×19 arcmin in diameter (0.52 pc); the outer torus is 12×22 arcmin in diameter (0.77 pc); and the outer-most ring is about 25 arcmin in diameter (1.76 pc). We see the outer-most ring as flattened on one side due to its colliding with the ambient interstellar medium.[11]

Expansion of the whole planetary nebula structure is estimated to have occurred in the last 6,560 years, and 12,100 years for the inner disk.[12] Spectroscopically, the outer ring's expansion rate is 40 km/s, and about 32 km/s for the inner disk.

Knots
A closer view of knots in the nebula

The Helix Nebula was the first planetary nebula discovered to contain cometary knots.[13] Its main ring contains knots of nebulosity, which have now been detected in several nearby planetary nebulae, especially those with a molecular envelope like the Ring nebula and the Dumbbell Nebula.[14] These knots are radially symmetric (from the CS) and are described as "cometary", each centered on a core of neutral molecular gas and containing bright local photoionization fronts or cusps towards the central star and tails away from it.[15] All tails extend away from the Planetary Nebula Nucleus (PNN) in a radial direction. Excluding the tails, each knot is approximately the size of the Solar system, while each of the cusp knots are optically thick due to Lyc photons from the CS.[3][6][16] There are about 40,000 cometary knots in the Helix Nebula.[17]

The knots are the probably result of the Rayleigh-Taylor instability. The low density, high expansion velocity ionized inner nebula is accelerating the denser, slowly expanding, largely neutral material which had been shed earlier when the star was on the Asymptotic Giant Branch.[18]

The excitation temperature varies across the Helix nebula.[19] The rotational-vibrational temperature ranges from 1800 K in a cometary knot located in the inner region of the nebula are about 2.5'(arcmin) from the CS, and is calculated at about 900 K in the outer region at the distance of 5.6'.[19]

Videos
This zoom sequence starts with a wide-field view of the rather empty region of sky around the constellation of Aquarius.
This video compares a new view of the Helix Nebula acquired with the VISTA telescope in infrared light with the more familiar view in visible light from the MPG/ESO 2.2-metre telescope at ESO's La Silla Observatory.

See also

References
  1. Su, K. Y. L.; Chu, Y.-H.; Rieke, G. H.; Huggins, P. J.; et al. (March 2007). "A Debris Disk around the Central Star of the Helix Nebula?". The Astrophysical Journal. 700 (2): L41–L45. arXiv:astro-ph/0702296. Bibcode:2007ApJ...657L..41S. doi:10.1086/513018. S2CID 15244406.
  2. "NGC 7293". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-12-07.
  3. O'Dell, C. R.; McCullough, Peter R.; Meixner, Margaret (2004). "Unraveling the Helix Nebula: Its Structure and Knots". The Astronomical Journal. 128 (5): 2339–2356. arXiv:astro-ph/0407556. Bibcode:2004AJ....128.2339O. doi:10.1086/424621. S2CID 119507454.
  4. Hora, Joseph L.; Latter, William B.; Smith, Howard A.; Marengo, Massimo (2006). "Infrared Observations of the Helix Planetary Nebula". The Astrophysical Journal. 652 (1): 426–441. arXiv:astro-ph/0607541. Bibcode:2006ApJ...652..426H. doi:10.1086/507944. S2CID 15427995.
  5. http://simbad.u-strasbg.fr/simbad/sim-ref?bibcode=2018yCat.1345....0G
  6. O'Dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; et al. (2002). "Knots in Nearby Planetary Nebulae". The Astronomical Journal. 123 (6): 3329–3347. Bibcode:2002AJ....123.3329O. doi:10.1086/340726.
  7. "Urban Legends Reference Pages". The Eye of God. Retrieved 2007-07-16.
  8. "The Eye of Sauron (aka NGC7293)". Sky and Telescope.
  9. Nancy Atkinson (4 October 2012). "Eye-Like Helix Nebula Turns Blue in New Image". Universe Today. Archived from the original on 14 July 2014.
  10. Young, K.;, Cox, P.; Huggins, P. J.; Forvielle, T.; Bachiller, R. (1999). "The Molecular Envelope of the Helix Nebula". The Astrophysical Journal. 522 (1): 387–396. Bibcode:1999ApJ...522..387Y. doi:10.1086/307639.CS1 maint: uses authors parameter (link)
  11. Henry, R. B. C.; Kwitter, K. B.; Dufour, R. J. (June 1999). "Morphology and Composition of the Helix Nebula". The Astrophysical Journal. 517 (2): 782–798. doi:10.1086/307215. ISSN 0004-637X.
  12. O'Dell, C. R.; McCullough, Peter R.; Meixner, Margaret (2004). "Unraveling the Helix Nebula: Its Structure and Knots". The Astronomical Journal. 128 (5): 2339–2356. arXiv:astro-ph/0407556. Bibcode:2004AJ....128.2339O. doi:10.1086/424621. S2CID 119507454.CS1 maint: uses authors parameter (link)
  13. "APOD: 2008 April 13 - Curious Cometary Knots in the Helix Nebula". apod.nasa.gov. Archived from the original on 2012-05-05.
  14. O’Dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; Burkert, A. (June 2002). "Knots in Nearby Planetary Nebulae". The Astronomical Journal. 123 (6): 3329–3347. doi:10.1086/340726.
  15. Huggins, Patrick; Bachiller, Rafael; Cox, Pierre; Forveille, Thierry (1992). "CO in the globules of the Helix nebula". The Astrophysical Journal Letters. 401: L43–L46. Bibcode:1992ApJ...401L..43H. doi:10.1086/186666.
  16. O'Dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; et al. (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). 15: 29–33. Bibcode:2003RMxAC..15...29O.
  17. Matsuura, M.; Speck, A. K.; McHunu, B. M.; Tanaka, I.; Wright, N. J.; Smith, M. D.; Zijlstra, A. A.; Viti, S.; Wesson, R. (2009-08-01). "A "Firework" of H2Knots in the Planetary Nebula NGC 7293 (The Helix Nebula)". The Astrophysical Journal. 700 (2): 1067–1077. doi:10.1088/0004-637X/700/2/1067. hdl:10355/5140. ISSN 0004-637X.
  18. Capriotti, Eugene R.; Kendall, Anothony D. (10 May 2006). "The Origin and Physical Properties of the Cometary Knots in NGC 7293". The Astrophysical Journal. 642 (2): 923–932. Retrieved 22 November 2020.
  19. Matsuura, M.; Speck, A. K.; Smith, M. D.; Zijlstra, A. A.; et al. (December 2007). "VLT/near-infrared integral field spectrometer observations of molecular hydrogen lines in the knots of the planetary nebula NGC 7293 (the Helix Nebula)". Monthly Notices of the Royal Astronomical Society. 382 (4): 1447–1459. arXiv:0709.3065. Bibcode:2007MNRAS.382.1447M. doi:10.1111/j.1365-2966.2007.12496.x. S2CID 118514953.

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