The Cygnus Loop is a large nebula in the constellation Cygnus, some 80 light-years across, lying about 2,500 light-years away and 330 light-years above the galactic plane, that is the remains of a supernova explosion inside another supernova explosion (see also supernova remnant); the most prominent part of it at visible wavelengths is the Veil Nebula. The initial supernova took place roughly 18,000 years ago and sent out powerful shock fronts that propagated into the interstellar medium, creating a giant cavity in the surrounding clouds. This was followed by the explosion of a second massive star, at least 5,000 years ago, shock waves from which are now interacting with the original cavity walls. The result of this interaction are ragged, sweeping filaments of denser interstellar matter that have been energized to the point at which they radiate at visible wavelengths, together with smoother, more delicate filaments, at the outer edges of the expanding shock fronts, formed where there the density of interstellar matter is much lower.