Helix Nebula
{{short description|Planetary nebula in the constellation Aquarius}}
{{For|the different but similarly named nebula|Double Helix Nebula}}
{{Redirect|Eye of God|the religious symbol|All-Seeing Eye of God|other uses}}
{{Planetary nebula
| name = Helix Nebula, NGC 7293
| type = Planetary
| image = File:NGC7293 (2004).jpg
| caption= NGC 7293 seen through several visible filters by Hubble Space Telescope
| credit =
| epoch = J2000
| ra = {{RA|22|29|38.55}}{{cite simbad
| title=NGC 7293
| access-date=2006-12-07}}
| dist_ly = {{val|200|1|ul=pc}} ({{val|650|3}})
| constellation = Aquarius
| radius_ly = 2.87 ly (0.88 pc)
| absmag_v =
| notes = One of the nearest PNe
| names = NGC 7293 Caldwell 63
}}
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, most likely before 1824, this object is one of the closest of all the bright planetary nebulae to Earth.{{cite journal
| author=Hora, Joseph L.
| author2=Latter, William B.
| author3=Smith, Howard A.
| author4=Marengo, Massimo
| title=Infrared Observations of the Helix Planetary Nebula
| journal=The Astrophysical Journal
| date=2006
| volume=652
| issue=1
| pages=426–441
| bibcode=2006ApJ...652..426H
| doi=10.1086/507944
|arxiv = astro-ph/0607541 | s2cid=15427995
}} The distance, measured by the Gaia mission, is 655±13 light-years.{{Cite web|url=http://simbad.u-strasbg.fr/simbad/sim-ref?bibcode=2018yCat.1345....0G|title=SIMBAD references}} It is similar in appearance to the Cat's Eye Nebula and the Ring Nebula, whose size, age, and physical characteristics are in turn similar to the Dumbbell Nebula, differing only in their relative proximity and the appearance from the equatorial viewing angle.{{cite journal
| author=O'Dell, C. R.
| display-authors=4
| author2=Balick, B.
| author3=Hajian, A. R.
| author4=Henney, W. J.
| author5=Burkert, A.
| title=Knots in Nearby Planetary Nebulae
| journal=The Astronomical Journal
| date=2002
| volume=123
| issue=6
| pages=3329–3347
| bibcode=2002AJ....123.3329O
| doi=10.1086/340726
| doi-access=free
}} The Helix Nebula has sometimes been referred to as the "Eye of God" in pop culture,{{cite web
| title=Urban Legends Reference Pages
| work=The Eye of God
| date=5 February 2004
| url=http://www.snopes.com/photos/space/eyeofgod.asp
| access-date=2007-07-16
}} as well as the "Eye of Sauron".{{cite journal
|url= http://www.skyandtelescope.com/online-gallery/the-eye-of-sauron-aka-ngc7293/
|title= The Eye of Sauron (aka NGC7293)
|journal= Sky and Telescope
|url = http://www.universetoday.com/97713/eye-like-helix-nebula-turns-blue-in-new-image/
|title = Eye-Like Helix Nebula Turns Blue in New Image
|author = Nancy Atkinson
|date = 4 October 2012
|publisher = Universe Today
|url-status = live
|archive-url = https://web.archive.org/web/20140714152714/http://www.universetoday.com/97713/eye-like-helix-nebula-turns-blue-in-new-image/
|archive-date = 14 July 2014
}}
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 Earth's perspective, 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 {{val|10600|+2300|-1200}} years, based on the ratio of its size to its measured expansion rate of 31 km·s−1.
Structure
File:Comets Kick up Dust in Helix Nebula (PIA09178).jpg{{cite journal | bibcode = 2007ApJ...657L..41S | title = A Debris Disk around the Central Star of the Helix Nebula? | date = March 2007 | journal = The Astrophysical Journal | volume = 700 | issue = 2 | pages = L41–L45 | doi = 10.1086/513018 | author = Su, K. Y. L. | display-authors = 4 | author2 = Chu, Y.-H. | author3 = Rieke, G. H. | author4 = Huggins, P. J. | author5 = Gruendl, R. | author6 = Napiwotzki, R. | author7 = Rauch, T. | author8 = Latter, W. B. | author9 = Volk, K. |arxiv = astro-ph/0702296 | s2cid = 15244406 }}]]
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). The outer-most ring appears flattened on one side due to it colliding with the ambient interstellar medium.{{Cite journal|last1=Henry|first1=R. B. C.|last2=Kwitter|first2=K. B.|last3=Dufour|first3=R. J.|date=June 1999|title=Morphology and Composition of the Helix Nebula|url=http://stacks.iop.org/0004-637X/517/i=2/a=782|journal=The Astrophysical Journal|volume=517|issue=2|pages=782–798|doi=10.1086/307215|arxiv=astro-ph/9901060|bibcode=1999ApJ...517..782H|issn=0004-637X|doi-access=free}}
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. Spectroscopically, the outer ring's expansion rate is 40 km/s, and about 32 km/s for the inner disk.
=Knots=
File:Close-Up of the Helix Nebula.jpg
The Helix Nebula was the first planetary nebula discovered to contain cometary knots.{{cite web|url=http://apod.nasa.gov/apod/ap080413.html|title=APOD: 2008 April 13 - Curious Cometary Knots in the Helix Nebula|website=apod.nasa.gov|url-status=live|archive-url=https://web.archive.org/web/20120505083443/http://apod.nasa.gov/apod/ap080413.html |archive-date=2012-05-05}} 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.{{Cite journal|last1=O’Dell|first1=C. R.|last2=Balick|first2=B.|last3=Hajian|first3=A. R.|last4=Henney|first4=W. J.|last5=Burkert|first5=A.|date=June 2002|title=Knots in Nearby Planetary Nebulae|url=http://stacks.iop.org/1538-3881/123/i=6/a=3329|journal=The Astronomical Journal|volume=123|issue=6|pages=3329–3347|doi=10.1086/340726|bibcode=2002AJ....123.3329O|doi-access=free}}
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.{{cite journal
| author=Huggins, Patrick
| author2=Bachiller, Rafael
| author3=Cox, Pierre
| author4=Forveille, Thierry
| title=CO in the globules of the Helix nebula
| journal=The Astrophysical Journal Letters
| date=1992
| volume=401
| pages=L43–L46
| bibcode=1992ApJ...401L..43H
| doi=10.1086/186666
}} 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.{{cite journal
| author=O'Dell, C. R.
| author2=McCullough, Peter R.
| author3=Meixner, Margaret
| title=Unraveling the Helix Nebula: Its Structure and Knots
| journal=The Astronomical Journal
| date=2004
| volume=128
| issue=5
| pages=2339–2356
| bibcode=2004AJ....128.2339O
| doi=10.1086/424621
|arxiv = astro-ph/0407556 | s2cid=119507454
| author=O'Dell, C. R.
| display-authors=4
| author2=Balick, B.
| author3=Hajian, A. R.
| author4=Henney, W. J.
| author5=Burkert, A.
| chapter=Knots in Planetary Nebulae
| title=Winds, Bubbles, and Explosions: A Conference to Honor John Dyson, Pátzcuaro, Michoacán, México, September 9–13, 2002
| editor-last1=Arthur
| editor-first1=Jane
| editor-last2=Henney
| editor-first2=William
| series=Revista Mexicana de Astronomía y Astrofísica, Serie de Conferencias
| publisher= Instituto de Astronomía, Universidad Nacional Autónoma de México
| date=2003
| volume=15
| pages=29–33
| bibcode=2003RMxAC..15...29O
The knots are probably the result of 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.{{cite journal |last1=Capriotti |first1=Eugene R. |last2=Kendall |first2=Anothony D. |title=The Origin and Physical Properties of the Cometary Knots in NGC 7293 |journal=The Astrophysical Journal |date=10 May 2006 |volume=642 |number = 2|pages=923–932 |doi=10.1086/501226 |bibcode=2006ApJ...642..923C |s2cid=120347309 |doi-access=free }}
The excitation temperature varies across the Helix nebula.{{Cite journal|author=Matsuura, M.|author2=Speck, A. K.|author3=Smith, M. D.|author4=Zijlstra, A. A.|author5=Viti, S.|author6=Lowe, K. T. E.|author7=Redman, M.|author8=Wareing, C. J.|author9=Lagadec, E.|display-authors=4|date=December 2007|title=VLT/near-infrared integral field spectrometer observations of molecular hydrogen lines in the knots of the planetary nebula NGC 7293 (the Helix Nebula)|journal=Monthly Notices of the Royal Astronomical Society|volume=382|issue=4|pages=1447–1459|arxiv=0709.3065|bibcode=2007MNRAS.382.1447M|doi=10.1111/j.1365-2966.2007.12496.x|doi-access=free |s2cid=118514953}} 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'.
Central star
File:HelixCentralStarLightCurve.png
The central star of the Helix Nebula is a white dwarf of spectral type DAO. It has the designations WD 2226-210, PHL 287, and GJ 9785. The star has a radius of {{convert|0.025|solar radius|km}}, a mass of {{Solar mass|0.678}}, a temperature of 120,000 Kelvin and has an apparent magnitude of 13.5.
A mid-infrared excess suggest a disk with a size of 35 to 150 AU, formed from Kuiper-belt like objects. The size was later revised to be a ring between 30 and 100 AU. The non-detection at longer wavelengths allowed a research team to reject a series of scenarios. The researchers think the mid-IR excess comes from a replenishment of dust particles from thousands of exocomets at high eccentricities, with an origin from an Oort cloud-like structure.
A 2024 study hypothesized that the central star might be orbited by a planet based on periodic variations in its light curve, but it cannot be ruled out that these variations are due to intrinstic stellar variability. Assuming an inclination of 25° (aligned with the nebula itself), this hypothetical planet is estimated to have a radius of {{convert|0.021|solar radius|km}}, or about 2.3 times the radius of Earth.
Another study from 2025 found from X-ray observation that the central star accretes the remains of a Jupiter-like planet. This is closer than the planet found via optical variability.
{{OrbitboxPlanet begin
}}
{{OrbitboxPlanet hypothetical
| exoplanet = c
| mass = 1
| semimajor = 0.004
| period = 0.12
| eccentricity =
| inclination =
| radius_earth =
}}
{{OrbitboxPlanet hypothetical
| exoplanet = b
| mass_earth =
| semimajor = 0.034
| period = 2.79
| eccentricity =
| inclination = ~25?
| radius_earth = ~2.3?
}}
{{OrbitboxPlanet disk
| disk = debris disk
| periapsis = 30
| apoapsis = 100
| inclination =
}}
{{Orbitbox end}}
Videos
{{multiple image
| align = left
| direction = horizontal
| image1 = Zooming into the Helix Nebula.ogv
| width1 = 300
| alt1 =
| caption1 = This zoom sequence starts with a wide-field view of the rather empty region of sky around the constellation of Aquarius.
| image2 = An infrared-visible light comparison of views of the Helix Nebula.ogv
| width2 = 300
| alt2 =
| caption2 = 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.
| image3 = Helix nebula.ogg
| width3 = 300
| alt3 =
| caption3 = A 3D model of the Helix Nebula from the Galaxy Map app (iOS/Android)
}}
{{Clear}}
See also
- New General Catalogue (NGC)
References
{{Reflist|30em|refs=
{{cite journal |last1=Iskandarli |first1=Leyla |last2=Farihi |first2=Jay |display-authors=etal |date=October 2024 |title=Novel Constraints on Companions to the Helix Nebula Central Star |journal=Monthly Notices of the Royal Astronomical Society |volume= 534|issue= 4|pages= 3498–3505|doi=10.1093/mnras/stae2286 |doi-access=free |arxiv=2410.03288 |bibcode=2024MNRAS.534.3498I}}
}}
External links
{{Commons category|Helix Nebula}}
- {{APOD |date=31 December 2009 |title=The Helix Nebula (NGC 7293)}}
- {{APOD |date=10 May 2003 |title=The Helix Nebula (NGC 7293)}}
- [http://www.nasa.gov/mission_pages/spitzer/multimedia/pia15817.html NASA/JPL-Caltech - The Helix Nebula (NGC 7293)]
- [http://messier.seds.org/xtra/ngc/n7293.html SEDS - The Helix Nebula (NGC 7293)]
- [http://www.nightskyinfo.com/archive/helix_planetary_nebula NightSkyInfo – The Helix Nebula (NGC 7293)]
- [http://www.snopes.com/photos/space/eyeofgod.asp Snopes - Helix Eye of God - Urban Legend]
- {{WikiSky|name=The Helix Nebula}}
- [http://www.constellation-guide.com/helix-nebula-ngc-7293-caldwell-63-in-aquarius/ Helix Nebula (NGC 7293) at Constellation Guide]
{{Aquarius (constellation)}}
{{Caldwell catalogue}}
{{NGC75}}
{{Portal bar|Astronomy|Stars|Outer space}}
{{Sky|22|29|38.55|-|20|50|13.6|695}}
{{DEFAULTSORT:Helix Nebula}}
Category:Aquarius (constellation)