From the Super Mario Wiki
A Pull Star is an object in Super Mario Galaxy and Super Mario Galaxy 2 that gives Mario yet another power that he never had before. It looks like a small, blue version of a Launch Star inside a blue bubble or orb. They can be found in virtually every galaxy between the two games. When the player touches one with the Star Cursor, it uses a blue gravitational aura to lift Mario off the ground, pulling him up toward it. Combining the power of these Pull Stars and using them in tandem can help Mario reach astounding heights or areas.
In some areas, Pull Stars are found orbiting other objects in the galaxy, such as Launch Stars or Power Stars. The player must use the right one at the right time to touch the object without accidentally "slingshooting" past it.
As with Launch Stars, Pull Stars will occasionally need to be assembled by gathering Star Chips. Five blue Star Chips are needed to make a Pull Star or set of Pull Stars appear. In Super Mario Galaxy, they were very common, but in the sequel, they're virtually nonexistent.
A single Pull Star appears in every Dome onboard the Comet Observatory in Super Mario Galaxy, where they allow Mario to view the galaxies that are accessible from that particular Dome. When he selects a galaxy to fly to, the Pull Star transforms into a Launch Star, which then shoots him to that galaxy. In Super Mario Galaxy 2, these were not needed, as Starship Mario simply travelled to each galaxy instead.
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