In the constellation of Gemini at a distance of 1500 light-years away lies an old planetary nebula called Abell 21, also known as the Medusa Nebula. It is over 4 light-years across and it was discovered by an American astronomer George O. Abell in 1955. There was some debate on its classification, some thought it might be a supernova remnant and others were supporting the theory of an old planetary nebula. In 1973 T. Lozinskaya from Shternberg Astronomical Institute measured the expansion velocity by means of interferometry and concluded that it must be a planetary nebula. The nebula is often described as crescent shaped, but in reality it has nearly spherical shape. The fainter hemisphere is much more diffuse thus giving an illusion of a crescent shaped object. Surface brightness of this nebula varies alot from +16 magnitudes all the way down to +25 in the faintest parts. For my telescope, this object is rather small and dim but Im still pleased how this turned out, even with a relatively short ~25 hour total exposure time. Maybe Ill revisit this object in the future if I invest in a bigger telescope some day.
