Hello! Is this an extremely long exposure shot or you combined multiple photographs in one? I found out that with too long exposures ( 20min-30min) there are quite some dead pixels over the photo. So, I prefer taking for example 60 shots with shorter exposure time over an hour and put them together in PS. Anyway, nice gallery, very impressive! Have a nice day!
In this case, because of the star-trails and light pollution in the area, its a stack of a lot of 30sec exposure shots, I use 'Starstax' software for the stacking, personally I get the best results with star-trails this way.
If you do one exposure you could also use the in camera noise reduction setting to counter hot pixels.