Star trails are a form of long exposure photography, it is one of my favourite forms of long exposure photography but can be extremely time consuming as it requires a lot of trial and error. Just so you can see what I am talking about bellow is an example image.
Long exposure photography can be quite time consuming because the longer you leave the shutter open the longer the camera needs to process it and you can never be certain that the shot is correct until you get it back. When shooting star trails it becomes even more difficult with the lack of light; meaning composition and focus can be very difficult to do, it is usually best to setup before dusk so you get it right.There are tons factors that can massively effect taking a star trail photo.
A shot of just the sky may be nice but it lacks a subject so often putting something in the foreground can add a point of reference making the image more interesting and give the shot scale.
Little or no Clouds
Clouds can add an interesting effect to the sky but they fog and diminish the brightness of the stars. Having a cloudless sky is a must and often star trails are clearest when you are high up (such as in the photo below).
There maybe no clouds in the sky but that does not mean it is clear, try and pick a night where the sky is really dark and there are no wisps of a forming cloud and little air pollution. Another consideration to take into account is the ambient light from streets and cars near by, often this can wash out an image. It is better to get away from a city centre or a road.
In order to take star trails you need a SLR camera with a Bulb mode (also called B). This allows the shutter to remain open until you let go of the shutter button (may work slightly differently on other cameras). Holding down the shutter button for over 30 minutes is a ridiculous idea so the purchase of a remote trigger is a necessity. I use a ML-L3 on my Nikon to take star trail photos, a remote tricker allows you to press its button once to star the shot and once to finish the shot. You will also need a stable tripod or a stand of some sorts that will hold the camera still. It can get very cold at night and the cold reduces battery life so go out fully charged up if you can.
Taking the shot
It is best to set this all up before darkness and make sure you are happy with your composition. Check you camera is firmly in place and your remote trigger works. When taking shots under bulb mode it is up to you to guess how long your shutter should be open so I advise that you start at 10 minutes (don’t be afraid to build up to over an hour) and build up until you get a clear shot. I would also suggest you take a notepad and pen and write down what time you started the shot and for how long for future reference.
Turn down the ISO to as low as it can go and put up the f stop this will reduce noise and let in as much light as possible. Another top tip is to check your cameras menu to see if it has an option for Long Exposure Noise Reduction if so turn this on and it will help remove noise you get from long exposures. Some cameras show the time the shutter has been open others however do not so a stop watch could be helpful. If it is still to light try and neutral density filter to darken the shot.
The method described here advocates taking one long exposure, however it is possible to do it by combining multiple shorter exposures in something like Photoshop.