The Medusa Nebula is a very old and large planetary nebula in the constellation of Gemini on the Canis Minor border. It was discovered in 1955, and until the early 1980s it was thought to be a supernova remnant.
The nebula lies at a distance of approximately 1000 light-years away with the 15.9 magnitude bluish star in the middle of the image above being responsible for this stunning and colourful display.
Measuring 12.67 by 10.25 arc-minutes in size - which is very large as compared to most planetary nebulae - the Medusa's surface brightness is very low, with surface magnitudes of between +15.99 and +25 reported. Because of this most websites recommend at least an 8-inch (200 mm) telescope with an [O III] filter to find this object although it is probably possible to image with smaller apertures. Abell 21 is best observed during winter when it lies overhead and a few degrees north of Procyon (mag 0.40).
I have used the following sources to compile the text above: http://www.perseus.gr/Astro-DSO-Nebulae-PN-Abell-21.htm, Wikipedia, the free encyclopedia; http://www.blackskies.org/mcneilpn1.htm;