Medusa Nebula, Abell 21 or SH2-274

Here's an image I took back in February as I haven't been able to muster any enthusiasm for processing the images.  My brother shamed me into working them.   Here is the Medusa nebula or Abell 21 or  SH2-274,  from inside the Gemini constellation.    The Medusa nebula is a planetary nebula.  For a long time the Medusa nebula was thought to be a supernova remnant but over time as the speed of its shell was measured, it was thought to be too low for a SN and now labeled a planetary nebula (PN).   PN are formed when an intermediate mass star, less than 8 times the mass of our sun, can no support nuclear fusion of material into heavier elements in its core, and unlike bigger stars which end in cataclysmic explosions called supernova, these stars just gently shed rings of ionized gas into the beautiful and sometimes complicated shapes we through our telelscopes.  What's left at the centers of planetary nebula are white dwarfs.  The medusa is thought to be about 4ly across and 1500ly distant from our solar system.   

I shot the Medusa with a TEC140 scope through Chroma 3nm narrow band filters plus R,G and B filters.   I captured 2 1/2 hours of each Ha, OIII and SII and 30 minutes each of red, blue and green for the stars.  I'm presenting it here as a HOO image and didn't use the SII.    I am rusty in my processing skills and am working to get them back up to snuff.