Medusa Nebula
Abell 21 or YM 29, also known as Medusa Nebula is a vast planetary nebulae with low surface brightness in the constellation Gemini. His own name from the mist the snake-like gaseous filaments, reminiscent to the hair of Medusa.
He was discovered in 1955 independently of each other by George Ogden Abell at Palomar Observatory and Hugh M. Johnson as part of the Yerkes - McDonald Survey. Until the 1970s has also been considered that it could be a supernova remnant. Based on the calculated rate of expansion and the thermal nature of the radio emission closed Soviet astronomers 1971 that the original classification is the most likely.
The object is not to be confused with the cluster of galaxies Abell 21 or the merging galaxies NGC 4194, which are sometimes placed with the head of Medusa in conjunction because of their appearance.