A few years ago I had the pleasure to record a title for Dean Collins Software Cinema on Photoshop CS4. While much of that information is getting a bit long in the tooth, Softare Cinema has been kind enough to post some of the more relavant training videos on You Tube. Below is some fine instruction (if I do say so myself) on understanding what a histogram is, and how to use it as a digital light meter.
What are Star Trails?
Simply stated, star trails are the streaks in a photograph left by stars during a time exposure as the Earth rotates. Objects in the foreground of the photograph remain sharp as they are not moving relative to the camera, while the stars, depending on which cardinal direction the camera is facing, will form concentric circles or streaks in the sky.