Capturing ‘star trails’ can be a very challenging yet rewarding task. My steps will provide correct methods that you can use to create a astounding image.
First you will need two key pieces of equipment. This includes: a camera that can take long exposure images and a tripod.
After collecting your equipment, find a spot away from street lights and light pollution. Next, mount your camera on your tripod and point it at the sky. For a star trail image that revolves around one star, direct your camera at the star “Polaris” since the earth rotates around that star. Next, set your camera to a 20-30 second shutter time, an f-stop of 5.6 or 8, ISO 800, and infinite focus mode. These settings are subject to change pending on your location, camera, and personal creativity. A similar method is described in this article: http://photoextremist.com/star-trails-tutorial
Now it is time to shoot. Since I have the Nikon D5100, I can set the interval timer so that the camera automatically takes a photo every X amount of time. If your camera does not have this mode, you will have to manually press the shutter release every X seconds.
When you have a sufficient amount of photos (200+) you can conclude your time taking photos. After importing the photos into photoshop, layer them on top of each other using the blending options (this video shows how to do this http://www.youtube.com/watch?v=tMERM7qQ_Ng)