These are very large (4096 x 4096 pixels) sets of alpha planes to be used as backgrounds for space scenes. Each alpha plane within a set is "edge-matched" on all four sides with four of the other planes so that they will form a continuous backdrop of very large proportions. An enormous backdrop is desirable if you are animating a space ship moving across the heavens, or if you have a complex scene full of spacecraft. Of course, you don't need to use any more than one of these alpha planes, if you wish.
The alpha planes can form a "box" so that you can shoot your scene entirely within the box - perhaps with a circling camera, or when you need several individual renders from different angles. If you form a box, the edges will not show in your renders.
There are three sets of alpha planes in this package, one with predominately blue-purple skies, one with predominately red stars and nebula, and one with predominately white nebula and blue stars. (And there is a "bonus" package of two single alpha planes.)
The images above display some representative alpha planes from each of the three packages. However, due to the need to "dumb down" pictures for the web (75% jpg quality), most of the small stars do not show up in these images and the images here are more "fuzzy" than they are in actual renders.