Image of a small portion of the Cygnus Loop supernova remnant, which marks the edge of a bubble-like, expanding blast wave from a colossal stellar explosion, occurring about 15,000 years ago. This Hubble Space Telescope image shows the structure behind the shock waves. The supernova blast is slamming into tenuous clouds of interstellar gas. This collision heats and compresses the gas, causing it to glow. A bluish ribbon of light stretching left to right across the picture might be a knot of gas ejected by the supernova. The Cygnus Loop appears as a faint ring of glowing gases about three degrees across (six times the diameter of the full Moon), located in the northern constellation, Cygnus, the Swan. The supernova remnant is within the plane of our Milky Way galaxy and is 2,600 light-years away.