The Medusa Nebula (or Abell 21) is a planetary nebula in a distance of 1,500 lightyears in the constellation Gemini. It was discovered fairly recently (1955) and it was first thought to be a supernova remnant. It has its name from the braided serpentine filaments of glowing gas which look like the serpent hair of the Medusa (from the Greek Mythology).
I had to use GradientXTerminator to remove some gradients in the Ha, SII and OIII images. I used a star mask to whiten the stars and Carboni's astronomy tools to add some fraction spikes.