Battlerock Galaxy: After rescuing Luigi from the haunted mansion, he'll start helping you find power stars. The second star he finds is on Battlerock. Go rescue him and get the star.

Buoy Base Galaxy: As soon as you get there, dive underwater and get the underwater missiles to hit the cage blocking the green pipe. Head down there, and you'll pop up on the bottom side of the plane. There will be missile launchers everywhere, so make sure you have full health and good jumping skills. The star will be inside. Get a missile to hit it and you get your second green star.

Dusty Dune Galaxy: On the second level of Dusty Dune, where you start, right underneath is a Hungry Luma. Feed him the ammount of star bits he wants, a new planet will form and a launch star will shoot you there. Now, you must collect all 5 silver stars. This part is like an elevator thing; you land on the button and you go up or down go in every direction you can until you have all 5 silver stars.