Binary star systems are two stars that orbit each other. Most stars in the Universe are part of a "multiple star system" - meaning two or more. Astronomers know this by breaking down the light sources (spectroscopy) from what looks like a single star - even in a powerful telescope.
Planets around multiple star systems (click for the youtube video) are now thought to be more prevalent than single star systems like our Sun (or "Sol"). So a "Star Wars" Universe and the fictional planet "Tatooine" with two suns setting is now way more likely. Wait for NASA's Kepler Spacecraft to see how many Earth-like planets are out there.