[bojan]

Actually, when I think of it, the image is pretty good, considering you took 6 sec exposures only.
The noise came from stretching.
You really need longer exposures..
However I can not explain those black artefacts.
The colours may be wrong because of light pollution? Where did you take those pictures from?

Just to give you a reference, the attached image was taken from my backyard last year, with MTO1100-A (1100mm, F11) lens, 30 sec at ISO1600.
It is a stack of 8 exposures.