Gateway Galaxy
Gateway Galaxy is the first galaxy encountered in Super Mario Galaxy.
This galaxy initially serves as the tutorial galaxy, getting the player acclimated with the basic controls of the game. Later, it is revisited in the introductory purple coin mission. It's one of two galaxies to have only two stars, the other being Buoy Base Galaxy.
List of Stars
Gateway Purple - Gateway's Purple Coins
Routing Info
Gateway Galaxy is re-accessible after the player has unlocked the Gate.
Boo's Boneyard Galaxy will only be accessible after the player has collected Gateway Purple, Ghostly 1, and fed the Hungry Luma next to the Gate 1200 star bits.
Completing Gateway Purple unlocks the Red Star within the Comet Observatory, which is a fast way to get from the ground floor to the Garden or Engine Room.