Though we have long nights, its a tad chilly right now to be outside doing star trail shots, so I confess this long exposure night scene was taken from the balcony in a rather warmer Tenerife. Many star trail shots are composite images made up of multiple photos of exactly the same scene blended together in post processing using software in the same kind of way that HDR images are, however it is possible to take a long exposure to create shorter star trails in-camera in just a single image.
It was a blustery evening and not really ideal conditions for taking razor sharp long exposures, but Tenerife is renowned for its clear skies, even being home to a well regarded observatory, so I simply couldn't resist the opportunity. I was particularly fortunate to have the pole star so visible and placed above the cliffs. It was amazing how many more stars became visible even in the 4 second photograph than could be seen with the naked eye.
In the first image you can see the starry sky as we humans see it, taken using a shockingly high ISO of 6400 and a 4 second exposure and then as the camera captured it over a period of 20 minutes (1236 seconds). Both shots used an aperture of f 4.0 and had long exposure noise reduction switched on. One other thing to remember is that it will take as long to store the image on completing the shot as it did to expose it in the first place, so this definitely isn't something to try in a rush.