Cygnus Loop

A circular shell of nebulosity with a diameter of 3° in the constellation Cygnus. The Veil Nebula forms one section of the circular structure. Radio emission confirms that the shell is in fact a supernova remnant. It is estimated to be 30,000 years old and to lie at a distance of 2,500 light years. The loop is still expanding at a rate of 6 arc seconds every hundred years, though this must be slower than the original expansion rate because of the braking effect of interstellar material.