Super Mario Galaxy Unlockables
Purple Comets
After you beat Bowser at the Galaxy Reactor and watch the credits, a message will say "The Purple Comet has appeared." Purple Comets make 100 (or sometimes 150) Purple Coins appear in the galaxy it is orbiting. Sometimes you are timed and sometimes you are not. These are the last 15 stars in the game and you should get them.
Submitted By: A Person
See Your Death Count
The Grand Finale Galaxy, which is the secret galaxy that can be unlocked after 120 Stars are gathered with both Mario and Luigi, holds another secret.
Once this final Star from the stage is acquired with both Mario and Luigi, you will be able to see how many times you died while playing through the game.
Submitted By: Dave
I can fly!
Entry Location:
The Gate
After you have beaten Bowser with Mario, go to the gate and step on to the little castle, go talk to Rosilina after, she will leave you with a red luma who will challenge you to collect 100 purple coins, if you collect every coin you will get a red star and the ability to fly around the main area.
Submitted By: gamefreakl1996
Grand Finale Galaxy and Star 121
Play through the game with Luigi and collect all 120 stars. Beating Bowser again unlocks Grand Finale Galaxy, which is the festival from the beginning of the game. To get star 121 collect 100 purple coins in this galaxy.
Unlock Luigi
Collect all 120 stars and fight Bowser again by speaking to Rosalina on the Comet Observatory. Wait for the credits to end and you'll see a message saying the Luigi is now playable.