Unlockable: 121st Star
There's actually 121 Stars in the game, not 120. To get it, you must first have all 120 Stars. This unlocks Luigi. Beat the entire game with Luigi, defeating Bowser at the end. After this, a new galaxy will be unlocked. Finish this final galaxy to get the 121st Star.
More tricks, tips and cheats for this game are right here - Super Mario Galaxy cheats
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