Well, it's not rocket science... and you're shooting digital so you don't have to worry about reciprocity failure. Find a good place out of the wind, set the camera on 'B' for a multi-second exposure, and have at it. If you have more or less complete darkness, you can have exposures of an hour or longer. According to Leica the M8 is threaded for a standard cable release, so you'll need one of those. After exposure, have a look at what your histogram/ viewfinder shows you. Being an M8 you don't have to worry about MLU. If you have a flashlight, see if you can get a red filter for it so it doesn't completely destroy your night vision. Also, if you are going to be turning on a light, try to keep one eye closed you only lose night vision in one eye. Of course, turn the light off between exposures. DOF only comes into play if you're shooting sky trails against an earth-based foreground. As you know, or might know, because of the earth's rotation stars show up as concentric arcs on long exposures. If you aim the camera at Polaris, you'll get concentric circles instead of just arcs as the earth's axis is more or less in line with that star. Also, different stars are different colours, so be prepared for white, red, yellow, green, blue... There's probably more, but trial and error is a good teacher.