Cygnus loop supernova remnant. True-colour optical image of the Cygnus loop supernova remnant in the constellation Cygnus. These faint wisps of gas are the remnants of a star which blew up around 20,000 years ago. It is composed of hydrogen gas which glows due to excitation from the pressure of the supernova's shock wave. The remnant lies about 2500 light years away. The brightest part of the ring is also known as the Veil Nebula (NGC 6992, upper left), and the ring as a whole is sometimes called the Cirrus nebula. The section at right is NGC 6960. This image was produced by digitally combining photographs taken by the Palomar Schmidt Telescope in blue and red light.