Although Abell 21, or Sh2-274, has a large apparent diameter of 10 arcminutes, its surface brightness is so low, with features ranging from magnitude 16-25, that it was not discovered until 1955. It is a planetary nebula lying at a distance of 1,500 light-years in the constellation Gemini, which gives it an estimated diameter of 4 light-years. The parent star, now a white dwarf, is thought to be the deep blue star near the center of the crescent. The braided filaments of the shell the star shed during its red giant phase, resemble the serpents that comprise the hair of the mythical character Medusa, giving the nebula its popular name, the Medusa Nebula. The braided shell cradles a sphere of gas glowing a soft blue, largely due to emissions by oxygen III. Beyond the beautiful braided crescent, additional hydrogen-alpha nebulosity, shed earlier by the star, can be seen at the center right and upper right corner of the image.
Telescope/Mount: Celestron Compustar C14 with AstroPhysics 0.75x focal reducer (f/8)
Camera: SBIG STL 11000M with Baader Planetarium HaRGB filters.
Location: Pommier Observatory, Portland, OR, USA.
Dates:2014-01-05 through 2014-03-09.
Exposures: HaRGB=1020:110:100:100=22 hours 10 minutes total exposure.