Editing Helix Nebula (section)

===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" />