Which figure are you using? The Unimesh line has some iris morphs but I don't think there's a star. The "Eyez" (at RuntimeDNA.com) are detailed eye models compatible with most of the popular figures, and they include an iris you can transmap to any shape you like. There were a couple of add-on texture sets that included some really whacky iris transmaps, I know there were several stars included. But that's a pay-for product; if you're looking to do this yourself I think Kirwyn's suggestion is probably the way to go: edit the iris texture you're using.