In amateur astronomy, the term Deep Sky is used to summarize those optically observable celestial objects that are outside the solar system (i.e. not asteroids, planets or comets), but are not individual stars. They include all star clusters, nebulae and galaxies.
Galactic nebulae are gas or dust regions that are stimulated to shine by hot stars (emission nebula, mostly red) or reflect their light (reflection nebula, mostly blue).
Because of the little light the stars give off, many pictures have to be taken for a single picture - the little light of the stars or galactic gas nebulae has to be collected in many pictures.
In order to compensate for the rotation of the earth during these long exposure times, the telescope / lens has to be tracked very precisely using a special mount.
These individual recordings are then combined on the computer using special Software (PixInsight, StarrySkyStacker, AstroPixelProcessor, ...) for the final astro record.