Helix Nebula

Helix Nebula
Nebula

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 ascension 22h 29m 38.55s[2]
Declination −20° 50′ 13.6″[2]
Distance 714+88
−68 ly (219+27
−21 pc)[3] ly
Apparent magnitude (V) +7.6[2]
Apparent dimensions (V) 25′[4]
Constellation Aquarius
Physical characteristics
Radius 2.87 ly (0.88 pc)[4] ly
Notable features One of nearest PNs
Designations NGC 7293[2] Caldwell 63

The Helix Nebula, also known as The Helix, NGC 7293, is a large 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.[5] The estimated distance is about 215 parsecs (700 light-years). 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]

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 a planetary nebula nucleus or PNN, 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 Helix Nebula in the constellation of Aquarius lies about 700 light-years away, spanning about 0.8 parsecs (2.5 light-years). Recent images by the Hubble Space Telescope of the Helix Nebula are a composite of newly released images from the ACS instrument and the wide-angle images from the Mosaic Camera on the WIYN 0.9-metre telescope at Kitt Peak National Observatory.

Currently, the age is estimated to be 10600+2300
−1200 years, based solely upon a measured expansion rate of 31 km·s−1.[6]

Structure

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.

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.[10] Spectroscopically, the outer ring's expansion rate is 40 km·s−1, and about 32 km·s−1 for the inner disk.

Knots

Closer view of knots in the nebula.

The Helix Nebula was the first planetary nebula discovered to contain cometary knots.[11] Its main ring contains knots of nebulosity, which have now been detected in many nearby planetaries. These knots are highly radially symmetric (from the PNN) and are described as "cometary", each centered on a core of neutral molecular gas and containing bright cusps (local photoionization fronts) towards the central star and tails away from it. All tails extend away from the PNN in a radial direction. Excluding the tails, they are (very approximately) the size of the Solar system, while each of the cusp knots are optically thick due to Lyc photons from the PNN.[4][6][12] There are more than 20,000 cometary knots estimated to be in the Helix Nebula.[13]

The excitation temperature varies across the Helix nebula.[13] 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 central PNN, calculated at about 900 K in the outer region at the distance of 5.6′.[13]

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. 657 (2): L41–L45. Bibcode:2007ApJ...657L..41S. arXiv:astro-ph/0702296 Freely accessible. doi:10.1086/513018.
  2. 1 2 3 4 "SIMBAD Astronomical Database". Results for Helix Nebula. Retrieved 2006-12-07.
  3. ↑ Harris, Hugh C.; Dahn, Conard C.; Canzian, Blaise; Guetter, Harry H.; et al. (February 2007). "Trigonometric Parallaxes of Central Stars of Planetary Nebulae". The Astronomical Journal. 133 (2): 631–638. Bibcode:2007AJ....133..631H. arXiv:astro-ph/0611543 Freely accessible. doi:10.1086/510348.
  4. 1 2 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. Bibcode:2004AJ....128.2339O. arXiv:astro-ph/0407556 Freely accessible. doi:10.1086/424621.
  5. ↑ 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. Bibcode:2006ApJ...652..426H. arXiv:astro-ph/0607541 Freely accessible. doi:10.1086/507944.
  6. 1 2 3 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.
  10. ↑ http://adsabs.harvard.edu/abs/2004AJ....128.2339O
  11. ↑ http://apod.nasa.gov/apod/ap080413.html
  12. ↑ 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.
  13. 1 2 3 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. Bibcode:2007MNRAS.382.1447M. arXiv:0709.3065 Freely accessible. doi:10.1111/j.1365-2966.2007.12496.x.

Coordinates: 22h 29m 38.55s, −20° 50′ 13.6″

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