Curious Cometary Knots in the Helix Nebula


	Curious Cometary Knots in the Helix Nebula

amgumen

4-13-2008 8:16 PM

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4-14-2008 10:04 AM

tabsey

And we worry if we are a little short or too tall.

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What causes unusual knots of gas and dust in 
<A href="http://www.noao.edu/jacoby/pn_gallery.html">planetary nebulas</A>?  
Seen also in the 
<A href="http://apod.nasa.gov/apod/ap060625.html">Ring Nebula</A>, the 
<A href="http://apod.nasa.gov/apod/ap030113.html">Dumbbell Nebula</A> and the 
<A href="http://apod.nasa.gov/apod/ap060709.html">Eskimo Nebula</A>, 
the knots' existence was not initially predicted and their origins 
are still not well understood.  
<A href="http://hubblesite.org/newscenter/archive/releases/1996/13/image/a/">Pictured above</A> is a fascinating image of the 
<A href="http://apod.nasa.gov/apod/ap060112.html">Helix Nebula</A> by the 
<A href="http://hubblesite.org/the_telescope/">Hubble Space Telescope</A> 
showing tremendous detail of its mysterious gaseous knots.  
The above cometary knots have masses similar to the Earth but have radii typically several times the orbit of Pluto.   
<A href="http://www.journals.uchicago.edu/doi/full/10.1086/501226">One hypothesis</A> 
for the fragmentation and evolution of the knots includes existing gas being 
<A href="http://en.wikipedia.org/wiki/Rayleigh-Taylor_instability">driven out</A> 
by a less dense but highly energetic 
<A href="http://en.wikipedia.org/wiki/Stellar_wind">stellar wind</A> 
of the central evolving star.  
The <A href="http://en.wikipedia.org/wiki/Helix_Nebula">Helix Nebula</A> 
is the closest example of a 
<A href="http://en.wikipedia.org/wiki/Planetary_nebula">planetary nebula</A> 
created at the end of the 
<A href="http://wind.cc.whecn.edu/~marquard/astronomy/sunlike.htm">life of a Sun-like star</A>.
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