When photographing stars, you can either get a star “field,” a static snapshot of the stars as points of light, or star “trails,” where the stars’ movements streak across the sky. How long you expose the image determines which you get. The first rule of thumb to remember is that the Earth rotates such that the light from a star begins to “move” after about 15 seconds. It’s apparent movement is largely dependent on your lens—the longer the focal length, the more apparent the movement; the wider angle lenses won’t show much movement till later because of the star point is so small. For comparison, a close-up photo of the moon can only be about 6 seconds before the Earth’s movement blurs it. Part of your experimentation will be to gauge the timing for how much “trail” you want.
I shot this pic at 9pm CST (about) using my 5D mark II with the 24-105 4L lens. Setting with on bulb mode using a wireless remote trigger. ISO 100, F5.6, SS 20 minutes!
