Abell 21, communally known as the Medusa Nebula(1), is a planetary nebula in the constellation Gemini(1) approximately 1500 light years distant(3). Serpentine filaments of glowing gas resemble the hair of the mythological Medusa(3). The nebula was originally discovered in 1955 by astronomer George Abell(2). Until the early 1970s the nebula was considered by many astronomers to be a supernova remnant(2). In 1971 Soviet astronomers concluded Abell 21 was most likely a planetary nebula(2).
The image above was captured using Ha (Hydrogen Alpha) and OIII (Doubly Ionized Oxygen) narrowband filters. The image was assembled by mapping Ha to red and OIII to green and blue. The stars were overlaid with RGB filtered data to give them a near true color appearance.