Complex sentence: The Veil Nebula constitutes the visible portions of the Cygnus Loop (radio source W78, or Sharpless 103), a large but relatively faint supernova remnant in the constellation Cygnus.
Split into simpler sentences: The Veil Nebula, is part of the Cygnus Loop, radio source W78, or Sharpless 103. It is a large, relatively faint supernova remnant in the constellation Cygnus.