The Cygnus Loop (radio source W78, or Sharpless 103) is a large supernova remnant (SNR) in the constellation Cygnus, an emission nebula measuring nearly 3° across. Some arcs of the loop, known collectively as the Veil Nebula or Cirrus Nebula, emit in visible light. Radio, infrared and X-ray images reveal the complete loop.
Read more about Cygnus Loop: Visual Components: The Veil Nebula, Southeastern Knot, Distance, Astronomical Ultraviolet Source, X-ray Source