Couldn't immediately find a good explanation of what STAR1 and STAR2 were. Looks like it's 256x256 bitmap that's scrollable and cycles automatically through a color palette. So you can make the stars "twinkle" by it just shifting through the palette

https://github.com/mamedev/mame/blob/master/src/mame/video/c...

edit: didn't see that "col" also contained an x-offset. So like (256*16)x256 but presumably "sparse", so I get more where the "star" idea comes from