Super Mario Galaxy 2: Unlock Green Prankster Comets for Green Stars
In Super Mario Galaxy 2 for the Wii, there are special green stars you can acquire after you beat the game.
To find them, you must first beat the game with 120 stars. If you already beat the game, you can still get them once you acquire 120 stars, but you must go back and beat bowser again to unlock them.
Once you do this, you will see a cutscene of Rosalina in the Library surounded by Lumas, telling a story an old story about the Green Lumas.
After the cutscene you’ll notice that green prankster comets are now scattered throughout the universe, which is what unlocks the green stars. There is one green star for every power star which will take your star count to 240 if you can collect them all, so travel through every galaxy again to find them.