Each still is comprised of hundreds of photographs stacked on top of one another. It takes an entire night to create one photograph in this way. You have to have a super computer to finish the whole process in Photoshop, so instead I use StrarStax. Fuck, I wish they paid me to say that but it is a free program.
There are a bunch of factors that can ruin your photograph like cloudy weather, rain, the moon or any other light that could fuck with exposure. Not to mention the man errors at the late-slash-early hours of your holiday made changing batteries and bumping the camera from sheer fatigue.