In Super Mario Galaxy 2 for the Wii, there is a hidden 2D galaxy that will test your abilities to their best. To unlock the Grandmaster Galaxy you need to collect all of the 240 initial stars, which is 120 regular stars and 120 green stars from the prankster comets.
Once you do this, the Grandmaster Galaxy will be unlocked in World S which has one star by default. Once you deposit 9999 star bits into the Banker Toad, you will unlock the second star in the Perfect Run (aka Daredevil Comet) making a total of 242 stars to collect in the game.
