Short tutorial about how I made this video using free software:
Taking pictures of the stars
Take lots of photos in manual mode with a long shutter speed (30-60 seconds) and intervals as short as possible. I used CHDK on an old Canon Powershot to shoot at intervals. Take a dark frame (by covering the lens with your hand for example) at the end; this can be used to subtract sensor noise.
Stack pictures
I used Python and PIL library to stack pictures on top of each other. The script I used can be found here. (I don't know if it still works). Put the .py file in the folder with the photos and run it. It can be slow.
Create video
You can use Virtual Dub to create an AVI file of the stacked photos.
