They use what's called "Stacking software." Stacking minimizes the ISO noise that accumulates in one long exposure. You effectively take a lot of long exposure shots (probably several hours) and the software "stacks" the many exposures into one. It automatically matches control points in the photo to align the many photos into one and does all the calculations needed to get the merged photo right. It is really amazing software.
It's widely used for nature photography. The slick thing is you can create a huge false but very real appearing depth of field. For small subjects like insects, you set your focus point for the initial shot, change the focus point (a few thousandths of an inch), take a shot, etc. Software controls everything - changing the focus point and taking the shots. You can stack thousands of images this way to get photos that are impossible due to optical physics limitations. Here's an example:
One popular tool is "Zerene Stacker." Here is a Gallery of Images taken with Zerene Stacker.