Golden Chomp

From the Super Mario Wiki

The Golden Chomp is a special Incoming Chomp in Super Mario Galaxy. It appears in the Gusty Garden Galaxy, where it plays a prominent role as the hidden star that can be found during The Gusty Garden's Gravity Scramble. It contains a Power Star inside, but Mario is unable to break it open without the use of a Rainbow Star, which can be obtained after collecting a series of ? Coins. This power-up allows the player to destroy the Chomp, revealing the Power Star.