the trial galaxy rock is on top of the observatory (or around it). you have to get the three green power stars and a green launch star will appear use it to get to the trial galaxy rock.
You get every green star.
there red
When you get 120 stars you see a green comet and scatters 120 green stars
120.
Bowser
throw
yes there is you might need help from a friend and walkthroughs/cheats.
you have to get every star, green star, prankster comet and comet medal
The observatory is in Green Bank.
You go to the Trial Galaxies and talk to the green star.
Super Mario Galaxy 2 has 242 stars in the game, just like the prequel. However, the green stars only appear after you get 120 stars.Green stars appear in every galaxy (except Grandmaster Galaxy). Usually there are 2-3 green stars in a galaxy. Now the objective is to find the hidden stars in the galaxy, which can prove quite challenging.After getting all the green stars, you end up with 240 stars, which will unlock the final galaxy: the aforementioned Grandmaster Galaxy. There are 2 stars in the galaxy, which will bring the total to a satisfying 242.So in summary, you can only get the green stars after having 120 stars.I hope that helped!- Ryanthelion