How to Make Star Trails
Painting with Light
In order to make star trails, you will need to have an SLR with an adjustable shutter speed. Set the camera up on a very sturdy tripod, and plug it into your Mac with a usb cable. If you have a Nikon, like me, download Sofortbild and open it. Make sure your SLR is on manual zoom adjustment, and zoom completely out and set focus to infinity. Obviously, aim the camera at the part of the sky you want to shoot, and make sure there is an abject in the camera's view to provided perspective (a house, trees, mountain, etc). Set the ISO to 800, and experiment with the shutter speed to get the right look for your pictures. The longer the shutter speed, the more light can enter the image center, and the more stars you are going to capture. Enable time lapse shooting, choosing any specific interval you wish. The shorted the interval between shots, the more likely you are going to get a smooth time lapse, or continuos star trails at the end.
Assembling the Star Trails
Using the free program called StarStax, import all the photos you took of the night sky, and make sure the image setting is on 'lighten'. This will then create your Star Trail, and will save it to your desktop
If you want to make a time lapse of the night sky, purchase Quicktime 7 and 'Open New Image Sequence'. Select how many frames per second you want for your video; the more fps, the smoother your time lapse will be. However, if you didn't take enough pictures, then choose a lower fps; your video will be jerky, but still pretty cool