The Cygnus Loop -  radio Source . W 78 , or .Sharpless , 103 - is a large supernova remnant . SNR . in the constellation Cygnus , an emission  nebula measuring nearly , 3 , across . N1. Some arcs of the loop , known collectively as the Veil Nebula or Cirrus Nebula emit in visible light . N2 . Radio , infrared and X - ray images reveal the complete loop