Most of these letters come after seeing these characters. Look around the galaxy to find the robot and he should ask you to help him. Help him and you will get a star.
Sometime later a letter will appear to get another star from the same character. This doesn't happen with all the characters though.
Complete boulder bowl galaxy
He is a large boulder dude that isn't difficult to kill at all. Pretty rad guy.
It's a hidden star and can be found by simply going to any level
super Mario galaxy 2
yes in super Mario galaxy 2 you get gobbbbleguut
Puzzle Plank Galaxy Boulder Bowl Galaxy Hightail Falls Galaxy Wild Glide Galaxy Cosmic Cove Galaxy Honeybloom Galaxy Bowser's Lava Lair Galaxy
Find out for yourself
Mario Galaxy 2 has been out for months.
Super Mario Galaxy happened in 2007.
No, there are no cheats in Super Mario Galaxy.
As long as Miyamoto lives, Mario will live. actually their making a new Mario galaxy called [suprise suprise] Mario galaxy 2
There's no Super Mario Galaxy 3.