Star Ball
The Star Ball is a large, transparent, capsule-like ball with a Power Star in it, that can be found in Super Mario Galaxy. It is used in Rolling Green Galaxy, Rolling Gizmo Galaxy and at the last part of the second Star mission in Melty Molten Galaxy.
The player can control the Star Ball by first jumping on top of it and tilting the Wii Remote in the direction they wish to travel while trying to move at a steady pace. Once Mario is on the Star Ball, it is impossible for him to dismount it.
In all three of its appearances, Mario's objective is to ride the Star Ball into a hole marked by a yellow flag, similar in appearance to a golf hole. To get to the flag, Mario must bypass numerous challenging obstacles, including narrow ledges, Goombas, Bob-ombs and moving platforms.
Once Mario rolls the Star Ball into the hole, the ball will explode, and the Power Star within will be released. The Power Star will move itself to the top of the flag in all three galaxies, where Mario must climb up in order to collect it.
They re-appear in the upcoming Super Mario Galaxy 2, most likely behaving the same, in Puzzle Plank Galaxy.