The Medusa Nebula is a planetary nebula 1,500 light-years distant in the constellation of Gemini. It is also known as Abell 21 and Sharpless 2-274. It was originally discovered in 1955 by George Abell, who classified it as an old planetary nebula. Until the early 1970s, the nebula was thought to be a supernova remnant. With the computation of expansion velocities and the thermal character of the radio emission, Soviet astronomers in 1971 concluded that it was most likely a planetary nebula. The Medusa's central star is the faint one near the center of the overall bright crescent shape. As the nebula is so large, its surface brightness is very low. The Medusa Nebula is estimated to be over 4 light-years across.

Sh2-274 - Medusa Nebula, which this image is part of.