Super Mario Galaxy/The Red Star, Purple Comets

The Red Star
The red star is a one of a kind star that is only found when you go back to the Gayeway Galaxy (the one where you have to find the three lumas) via gate on platform after engine room. When you go back, go on the big structure in the middle to find Rosilinia. After a bunch of talking, a red luma comes and gives you a red star that transforms you in to Cosmic Mario. You can now fly when you jump then spin, use this skill to collect 100 purple coins. Once you do, the luma gives you the red star, and you can use the red star in the Observatory.

Purple Comets
Once you beat the game, Purple Comets start orbiting space making it so when you go in the galaxy, you need to collect 100 purple coins. Collect them all and you will get another star. Now you can get all 120 (until you do the Luigi story…)