1 more green star

One you get from Luigi (you obviously have that one). Another is in a dome, past some Bullet Bills, and through a pipe under the water in the Buoy Base Galaxy. The last is in a secret pyramid that you can access by going to the second level of the Dusty Dune Galaxy. Now, when you get to the part with the brown ring with the rocks going around in circles, go to the underside of the platform that you land on and feed the Hungry Luma.



Hope it helps!!!!

You could be more specific on which stars you already have, but anyway here are the locations:

Buoy Base Galaxy: choose the mission with the ? Mark, then go underwater and make a torpedo ted (yes, torpedo ted, it's his official name)crash into a cage containing a green pipe. Enter it and then make a bullet bill follow you until it crashes into the next cage which contains a green star

Dusty Dune Galaxy: choose the mission with the ? Mark and play normally (well only make sure you collect a lot of starbits) until you get to a planet with a brown ring and 4 rocks rolling on it. One quick tip: on the underside of this planet is an rainbow star, grab it and crush the rocks to collect even more starbits. You may be wondering, why does this crazy guy makes me collect so many starbits? Here's the reason: in that planet's underside, there's a hungry luma. Feed it and go to the new planet, a pyramid like planet. Here you'll have to collect 5 silver stars to make the green star appear.

Battlerock Galaxy: choose the mision with the ? Mark, again. Advance until you see the power star you have already collected, but don't grab it!!! Instead guide a bullet bill into a cage that contains a luigi (and I say a luigi because I've seen more that one together) under the saucer(lol, actually, that's the name of the mission). To be more specific, it's the saucer you jump to when the moving saucer stops.
He will give you a green star, and also will ask you to say rosalina that he found it and not you... But who listens to what luigi says anyway? I hope this helps!