But galaxies are not created only by stars, the space among stars is not empty, despite the matter here so thin that nobody would hesitate to call it “perfect vacuum” (we intentionally omit dark matter here). And still, despite the hydrogen atoms are so sparse, long exposure astrophotography shows their collective glow as a red background where we would expect black emptiness, like on the wide-field image of a part of constellation Cygnus by Paolo Moroni (notice the “Crescent” nebula in the left part of the image).

Paolo used his G2-8300 camera with Samyang 135 mm F/2 lens and Sky-Watcher ED80 telescope with 0.85× Focal Reducer. Total exposure time reached 30 hours.