The name comes from the fact that Remus is a werewolf and there is a star called 'Sirius'. Other common names for this pairing are Remus/Sirius, Puppyshipping and The One True Way.
Person: What, Remus and Sirius? Together? Really? You're forgetting that Remus marries Tonks.
Shipper: HE ONLY MARRIES HER AFTER SIRIUS DIES HE AND SIRIUS WERE SO TOTALLY IN LOVE IT'S THE SEXIEST THING EVER SIRIUS HAD A FLYING MOTORBIKE
Person: I'm willing to believe Sirius had a little bromance with James Potter-
Shipper: NOOOOOOO WOLFSTAR IS CANON WOLFSTAR IS BEAUTIFUL