Play As Luigi in Super Mario Galaxy

Are you tired of playing as Mario? This will teach you how to unlock Luigi as a playable character in Super Mario Galaxy.

Steps

Defeat Bowser for the last time in the center of the universe. You must have 60 or more stars to do this. Don't worry, it would not be gone forever!
After doing this. a new number will appear on the floor next to Rosalina. If you do this AT 60 stars, at 61, the number would say, "59".
Collect the rest of the Stars (there are 120 stars to collect) so that the number next to Rosalina is 0 again.
After getting all of the Stars, talk to Rosalina and ask to fight Bowser again.
After the credits, you will get a message saying that Luigi is now a playable character.
Now you will be able to play as Luigi! Caution: When you just got Luigi, the star count would drop to zero on Luigi so you can do it all over with Luigi!

After collecting 120 stars with Luigi, you unlock the Finale Galaxy! This galaxy is a Purple Coin version of the Star Festival! Beat it with both Mario and Luigi for a total of 121 Stars!
Luigi is the same as Mario, except he jumps higher and runs slower. However, the Cosmic races are harder and Luigi has a hard time stopping.
Since Luigi can't stop as fast as Mario, jumping across small platforms can be tough if you can slide off, so take them nice and steady.