Moravian instruments, Inc., source: https://www.gxccd.com/art?id=584&lang=409, printed: 26.1.2020 19:12:12
|Is the Universe empty or full of stars and nebulae? This depends on what empty means to individual people. If you imagine a star like a small water drop, another water-drop sized star will be tens or hundreds of kilometers away. And this is valid only for galaxies — rather isolated islands of matter millions of light years apart. Space among galaxies lacks even these water drops.|
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.