I tried some star trails this summer with not really the greatest results. Its hard to gauge what your exposure should be and takes more luck than skill to get a good result. I went out last night on Highway 40 west of Cochrane to try again and the results, while varied, were more satisfying. My first attempt was a 30 minute exposure which was far too long for the aperture I had chosen. This attempt was a 7.5 minute exposure at f/2.8 and ISO 200.
I tried some shorter exposure times (1600 ISO and f/1.8) and still had about a 24 second exposure. Its astounding how much the stars move even in that short of time. (The larger size really show the motion)