>>2670725The principle of noise reduction when stacking is that your noise is random on every image. The real signal (stars, gas clouds..) is not random. When your stacking software add all these frames together, the real signal will be increased, and all the random elements (electronic noise, sky glow etc) will be "averaged out". Again, it works better if you have large stacks of images, that's why I suggested 100. Thats what I use for deep sky, but maybe its a little extreme for milky way shots
Dark frames are just black images (taken with the lens cap on with the same camera settings), that you add to your stacking software. These dark frames have only your camera noise in them, so the software knows better what to remove in your final image. Look it up on google, it really helps removing noise.
By light painting I mean add some light in your foreground (rocks or trees) in one of your frames (with a flash or any other light source) Then in photoshop add this foreground above your stacked stars picture. (or else your foreground will be blurry)
My typical exposure is 60 to 90 seconds long before having star trails. But I use a special mount for telescopes, with a motor in them that "follow" my object in the sky. With your standard tripod its more like 20 to 30 seconds (500/focal length)
Again, start with small stacks of images in DeepSkyStacker and look the difference in signal and noise. When you understand the method, start working with more images.
small tutorial on shooting the andromeda galaxy with stacking:
https://www.youtube.com/watch?v=e0JSTF8SGi4 