Sirius is not a single star at all, but a binary system of two stars. Polaris, the north star, is actually a system of three stars. And Castor, in the constellation of Gemini, actually consists of a whopping six stars. Current models show stars forming by the fragmentation of massive interstellar gas clouds, spinning themselves by gravity into stardom, in isolation and unaffected by nearby stars. But the plethora of systems in our galaxy that consist of multiple stars now has astronomers wondering why stars so often come in groups.