I've always been fascinated by photography that captures motion with streaks of light. But there is something special about star trails, the ability to turn movement imperceptible to the eye into something beautiful. Technically, the stars aren't moving of course (well, they are, but not THAT fast). It's the earth's rotation that makes them move across the sky, which is also why they appear to rotate around Polaris, the north star. Star trails are perhaps the simplest form of astrophotography, provided you don't live in New York City. I'm willing to bet we have more streaks of light caused by airplanes than stars. That would actually be a pretty cool experiment, I'll try that one day.
The equipment for star trails is pretty simple too: camera, intervalometer, tripod, dark skies and software. In the ancient days of film, i.e. fifteen years ago, you could simply set up the camera, open the shutter and let the film expose for hours. But, the sensors in today's cameras don't handle being used continuously for hours at a time. The picture quality degrades and you could probably damage the sensor. The way around this is to take a series of back-to-back shorter exposure and combine them with post-processing software. You can either take a large number of shorter exposures, or a smaller number of longer exposures. The latter is the easiest method, the fewer the files, the less storage and post processing. That being said, I generally use the former method, because it allows me the opportunity of doing a time-lapse of the stars moving across the sky, if I want. Two birds, one stone. Or hundreds of stones, I guess.
Setting up that series of back-to-back exposures is where the intervalometer comes into play. It is just a device that tells the camera to take N exposures every X seconds, each exposure is Y seconds long. If I want to track the movement of the stars across a period of 4 hours (14,400 seconds), decide your exposure (4 minutes or 240 seconds), divide to find the number of exposures (60). In an ideal world, you can set the same time for exposure and interval, but in my experience (with Nikon at least) it's better to set the interval 1 second longer (241 seconds). Yes, this will result in slightly more than 4 hours, close enough. Try it with the shorter exposure of 20 seconds, which is what I generally use for star time-lapse. Holy memory card Batman!!
Before starting, it's best to go into the camera settings and turn off image review. Without the LCD lighting up after every exposure, the camera will conserve battery power, crucial when it's cold out. Protect the camera and lens with a plastic bag in case of any surprise rain (uh, check the weather of course) and relax. So, now the camera will just be doing it's own thing for the next 4 hours. One thing to watch out for is the condensation that will form on the camera if left out through the morning. Condensation forms when an object is colder than the surrounding air, which is almost always the case in the morning when air heats up quicker than solid objects. I found my camera covered in frost after taking the first series of photos because condensation formed and then froze. Oops.
Now onto post-processing dozens, or hundreds, of images into a star trail. Make the same general edits to ALL images. Lightroom is indispensable for this operation. Then bring them into Photoshop, or similar program, as a layer stack and change the blend mode to lighten. Ta-da!! The lighten blend mode examines each pixel and chooses the brightest value from all layers. Since stars are white, they take "priority" over the darker sky.
I like this image because the stars almost form a funnel leading towards Polaris. It also shows that Polaris isn't perfectly north, it is also rotating. All of these images were taken several years ago about 20 miles north of Baltimore, where I grew up. Just far enough away that a good number of stars are visible, but not far enough away to be rid of the yellowish glow in the sky caused by the city. Whenever you have a photograph without pleasing colors, turn it B+W!!