These filaments of gas and dust visible in ultraviolet light were heated by the shockwave from a supernova which is still spreading out from the original explosion. The supernova which occurred between 5,000 and 8,000 years ago would have been bright enough to be seen clearly from Earth with the naked eye.
This ultraviolet image of the Cygnus Loop Nebula was taken by the Galaxy Evolution Explorer. The Cygnus Loop extends across an area more than three times the size of the full moon in the night sky and is tucked next to one of the “wings” in the constellation of Cygnus the Swan.
Image Credit: NASA