Last night I did my first attempt at making star trails. These is one of the shots that must be tried at least once by anyone doing astrophotography/nightskapes. This shot is made by allowing the rotation of the Earth to cause shifts in the apparent position of the stars (rise -> set).
I hit a few bumps, but still the end result was satisfying for a first attempt. Limited by my current gear, each of my subs are taken at the standard 30 second exposure time (I have no shutter-release cable or a shutter remote to maximize the bulb setting). My Nikon D3100 also takes an automatic dark frame subtraction after each exposure, so there is a delay in between the exposures (seen as gaps between each light frame). I learned that you can trick the camera to skipping the dark frame subtraction by turning off the DSLR after each exposure. However, my camera has been acting up and I also wanted all unnecessary contact to be avoided with my setup (I accidentally hit my tripod twice, resulting to the two large gaps in every trail).
The end result makes use of 51 subs totaling to 25.5 mins of exposure. Each of the light frames are manually calibrated in GIMP to produce a clean image. You may use available reduction software from the net to do the calibration for you.