Super Mario Galaxy: Galaxy of Trials Explanation
Some of you might have noticed the transparent green star over by the garage. This star will continue to be transparent unless you start getting those green stars! The green star leads to an area called the Galaxy of Trials. The Galaxy of Trials has 3 trails for you to try out. They are the hardest, from my standpoint and are a difficult challenge to those finding the game too easy for them. You might have even noticed the galaxy as you walk around, it is the strange shaped planet in the sky. This might not seem like an important place, but in order to get all 120 power stars you'll need to get the 3 power stars from this galaxy before you can complete the game.
Green Star Locations
1. The first and probably the easiest green star to get will be the one in Rumble Rock Galaxy. You will need Luigi to trigger it though, so you need to rescue him first if you haven't done so. After you get the picture of him stuck in Rumble Rock Galaxy proceed to the Galaxy and pick the first mission. Simply continue with the mission and when you get to the end by the star, instead of getting the star lead a bullet to the underside of the same platform and break Luigi out. He will then give you the green star and you can continue your hunt.
2. The second [star] is pretty simple too. Go to the Kitchen Dome and go to the Buoy Base Galaxy. Dive under water and lead a bullet to the bubble with the green pipe underneath it. After you break the bubble proceed going down the tube. After you go through the pipe, avoid the bullets and grab the star. Another simple star to get.
3. The last and final star is again pretty easy. You need to go to Dusty Dune Galaxy, in the Bedroom Observatory. The second mission is the one that will trigger it. Proceed with the mission as normal. Until you get to the planet with boulders going around a doughnut shaped planet. I advise you to go to the center of the doughnut then grab the hidden star underneath. You won't be able to see where you are going, but you will know when you get the star. Then, simply run into the boulders and grab their star bits. Then once you have 20 star bits go to the oval shaped planet nearby and go underneath it. Feed the Luma and then go to the planet created. You will need 5 silver stars to create the green star and complete the mission.
Good luck in the Galaxy of Trials! :D