Super Mario Galaxy Tips

Green Stars
In the game, there are three green stars that are found in addition to the regular ones. Once you have all three, a green launch star will appear near the garage (it will not appear until you have all three). It launches you to a small planet, where three minigame galaxies can be accessed (These are harder than normal). The locations of the green stars are:

Dusty Dune Galaxy - Episode 2 - Feed a hungry Luma under the planet you blast to after defeating the red Pokey.
Buoy Base Galaxy - Episode 1 - Have a torpedo follow you into a caged warp pipe near the edge of the tank while underwater.
Battlerock Galaxy - After you save Luigi, he will eventually go here (the Mailtoad in the front of the ship near Princess Rosalina will notify you) and he will be trapped. Follow the course until the end, and get a Bullet Bill to follow you to a caged Luigi under the nearby platform.