The Cygnus Loop
This supernova remnant in the constellation Cygnus, known as the Cygnus Loop, is roughly 20,000 years old. It is generally circular in shape except for a break-out towards the south. In addition, there are many bright, filamentary structures. The HRI image shows the angular structure of the Cygnus Loop. Even with this image some of the fine-scale structure is not resolved (both because of the coarse angular binning and through insufficient event statistics).