Optical photograph of the Veil Nebula (Cirrus Nebula, NGC 6979), part of an enormous annular supernova remnant which lies in the constellation of Cygnus. The remnant, known as the Cygnus Loop, is a circle of wispy filaments of gas and dust over three degrees in diameter. The loop is a giant expanding shell of gas, the remnant of a catastrophic stellar explosion which must have occured 30 to 40 thousand years ago.