Super Mario Galaxy 2/Rolling Coaster Galaxy

Star 1: The Rainbow Road Roll
The Comet Medal is along the path where the sign says "Experts stay right".

Star 2 (Prankster Comet): Purple Coins on the Rainbow Road
You need to collect 100 Purple Coins out of a possible 110. You have a plentiful amount of time, so feel free to go back to any coins that you missed. If you are having great difficulty with this mission, the cosmic spirit will appear after you have lost a certain amount of lives.

Green Star 1
If you do not get this green star in one shot you will have to restart the level and try again. When the pathway splits in two, take the left path. There should be a bump in the path. Use all of your speed and jump when on the top of the bump to reach the green star.

Green Star 2
This green star will be visible on the game screen. When the rainbow road get closer to the end of the level and is about to turn skinny, jump out of the right corner and fall into the green star.