View source for 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}}<ref name="simbad">{{cite simbad
  | title=NGC 7293
  | access-date=2006-12-07}}</ref>
| dec = {{DEC|-20|50|13.6}}<ref name="simbad" />
| dist_ly = {{val|200|1|ul=pc}} ({{val|650|3}})
| appmag_v = +7.6<ref name="simbad" />
| size_v = 25′<ref name="ODelletal2004" />
| constellation = [[Aquarius (constellation)|Aquarius]]
| radius_ly = 2.87 ly (0.88 pc)<ref name="ODelletal2004" />
| absmag_v =
| notes = One of the nearest PNe
| names = [[New General Catalogue|NGC]] 7293<ref name="simbad" /> [[Caldwell catalogue|Caldwell]] 63
}}
The '''Helix Nebula''' (also known as '''NGC 7293''' or '''Caldwell 63''') is a [[planetary nebula]] (PN) located in the [[constellation]] [[Aquarius (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.<ref name="Horaetal2006">{{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
 }}</ref> The distance, measured by the ''[[Gaia (spacecraft)|Gaia]]'' mission, is 655±13 light-years.<ref>{{Cite web|url=http://simbad.u-strasbg.fr/simbad/sim-ref?bibcode=2018yCat.1345....0G|title=SIMBAD references}}</ref> 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.<ref name="ODelletal2002">{{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
  }}</ref>  The Helix Nebula has sometimes been referred to as the "Eye of God" in [[pop culture]],<ref>{{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
}}</ref> as well as the "[[Eye of Sauron]]".<ref>{{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
}}</ref><ref>{{cite news
 |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
}}</ref>

==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 (constellation)|Aquarius]], and lies about 650 [[light-year]]s away, spanning about 0.8 [[parsec]]s (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&nbsp;km·s<sup>−1</sup>.<ref name="ODelletal2002" />

==Structure==
[[File:Helix3D.png|thumb|right|A 3 dimensional map of carbon monoxide in NGC 7293<ref>{{cite journal | bibcode = 1999ApJ...522..387Y | title=The Molecular Envelope of the Helix Nebula | journal=The Astrophysical Journal | volume=522 | pages=387–396 | year=1999 |author1=Young, K. |author2=Cox, P. |author3=Huggins, P. J. |author4=Forveille, T. |author5=Bachiller, R. | issue=1 | doi=10.1086/307639| doi-access=free }}</ref>]]

[[File:Comets Kick up Dust in Helix Nebula (PIA09178).jpg|thumb|left|Structure and cometary knots are prominent in this Infrared false-color image taken by the [[Spitzer Space Telescope]]<ref name="Suetal2007">{{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 }}</ref>]]

[[File:NGC7293Location.png|thumb|left|The location of NGC 7293 (labelled in red)]]

The Helix Nebula is thought to be shaped like a [[prolate spheroid]] with strong density concentrations toward the filled disk along the [[equator]]ial 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&nbsp;pc); the outer torus is 12×22 arcmin in diameter (0.77 pc); and the outer-most ring is about 25&nbsp;arcmin in diameter (1.76&nbsp;pc). The outer-most ring appears flattened on one side due to it colliding with the ambient [[interstellar medium]].<ref>{{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}}</ref>

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.<ref name="ODelletal2004" /> Spectroscopically, the outer ring's expansion rate is 40&nbsp;km/s, and about 32&nbsp;km/s for the inner disk.

===Knots===
[[File:Close-Up of the Helix Nebula.jpg|thumb|A closer view of knots in the nebula]]

The Helix Nebula was the first planetary nebula discovered to contain [[cometary knot]]s.<ref>{{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}}</ref> 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|Ring nebula]] and the [[Dumbbell Nebula]].<ref>{{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}}</ref>

These knots are radially symmetrical (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.<ref name="Huggins1992">{{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
}}</ref> 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 photon]]s from the CS.<ref name="ODelletal2004">{{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
 }}</ref><ref name="ODelletal2002" /><ref name="ODelletal2003">{{cite book
  | 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
}}</ref> There are about 40,000 cometary knots in the Helix Nebula.<ref>{{Cite journal|last1=Matsuura|first1=M.|last2=Speck|first2=A. K.|last3=McHunu|first3=B. M.|last4=Tanaka|first4=I.|last5=Wright|first5=N. J.|last6=Smith|first6=M. D.|last7=Zijlstra|first7=A. A.|last8=Viti|first8=S.|last9=Wesson|first9=R.|title=A "Firework" of H2Knots in the Planetary Nebula NGC 7293 (The Helix Nebula)|date=2009-08-01|url=http://stacks.iop.org/0004-637X/700/i=2/a=1067?key=crossref.e2bfcad2bbe1c7202b459debb74936a6|journal=The Astrophysical Journal|volume=700|issue=2|pages=1067–1077|doi=10.1088/0004-637X/700/2/1067|arxiv=0906.2870|bibcode=2009ApJ...700.1067M|issn=0004-637X|hdl=10355/5140|s2cid=119252556 |hdl-access=free}}</ref>

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]].<ref>{{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 }}</ref>

The [[excitation temperature]] varies across the Helix nebula.<ref name="Matsuuraetal2007">{{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}}</ref> The rotational-vibrational temperature ranges from 1800 [[kelvin|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'.<ref name="Matsuuraetal2007" />

==Central star==
[[File:HelixCentralStarLightCurve.png|thumb|left|A light curve for the Helix Nebula central star, adapted from Iskandarli et al. (2024)<ref name="Iskandarli2024"/>]]
The central star of the Helix Nebula is a [[white dwarf]] of spectral type DAO.<ref name="Iskandarli2024"/> It has the designations WD 2226-210, PHL 287, and GJ 9785.<ref name="simbad"/> 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.<ref name="Iskandarli2024"/>

A mid-infrared excess suggest a disk with a size of 35 to 150 AU, formed from [[Kuiper belt|Kuiper-belt]] like objects.<ref name="Su2007"/> 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.<ref name="Marshall2023"/>

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 intrinsic 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 [[Earth radius|radius of Earth]].<ref name="Iskandarli2024"/>

Another study from 2025 found from X-ray observation that the central star may be accreting the remains of a Jupiter-like planet. This would be closer than the planet found via optical variability.<ref name="Estrada-Dorado2025"/>

{{OrbitboxPlanet begin
| table_ref = <ref name="Iskandarli2024"/><ref name="Estrada-Dorado2025"/><ref name="Marshall2023"/>
}}
{{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 = <!--Inclination from the plane of the sky, in degrees-->
}}
{{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=

<ref name="Iskandarli2024">{{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}}</ref>

<ref name="Su2007">{{Cite journal |last1=Su |first1=K. Y. L. |last2=Chu |first2=Y. -H. |last3=Rieke |first3=G. H. |last4=Huggins |first4=P. J. |last5=Gruendl |first5=R. |last6=Napiwotzki |first6=R. |last7=Rauch |first7=T. |last8=Latter |first8=W. B. |last9=Volk |first9=K. |date=2007-03-01 |title=A Debris Disk around the Central Star of the Helix Nebula? |url=https://ui.adsabs.harvard.edu/abs/2007ApJ...657L..41S/abstract |journal=The Astrophysical Journal |volume=657 |issue=1 |pages=L41–L45 |arxiv=astro-ph/0702296 |bibcode=2007ApJ...657L..41S |doi=10.1086/513018 |issn=0004-637X}}</ref>

<ref name="Marshall2023">{{Cite journal |last1=Marshall |first1=Jonathan P. |last2=Ertel |first2=Steve |last3=Birtcil |first3=Eric |last4=Villaver |first4=Eva |last5=Kemper |first5=Francisca |last6=Boffin |first6=Henri |last7=Scicluna |first7=Peter |last8=Kamath |first8=Devika |date=2023-01-01 |title=Evidence for the Disruption of a Planetary System During the Formation of the Helix Nebula |journal=The Astronomical Journal |volume=165 |issue=1 |pages=22 |arxiv=2211.02251 |bibcode=2023AJ....165...22M |doi=10.3847/1538-3881/ac9d90 |doi-access=free |issn=0004-6256}}</ref>

<ref name="Estrada-Dorado2025">{{cite journal|arxiv=2412.07863 |last1=Estrada-Dorado |first1=S. |last2=Guerrero |first2=M. A. |last3=Toalá |first3=J. A. |last4=Maldonado |first4=R. F. |last5=Lora |first5=V. |last6=Vasquez-Torres |first6=D. A. |last7=Chu |first7=Y. -H. |title=Accretion onto WD 2226$-$210, the central star of the Helix Nebula |journal=Monthly Notices of the Royal Astronomical Society |date=2024 |volume=536 |issue=3 |page=2477 |doi=10.1093/mnras/stae2733 |doi-access=free |bibcode=2025MNRAS.536.2477E }}</ref>

}}

==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)]]
[[Category:Articles containing video clips]]
[[Category:Caldwell objects|063b]]
[[Category:NGC objects]]
[[Category:Planetary nebulae]]