How do you get to the hidden star in Boulder Bowl Galaxy

I've been trying to collect as many stars as I can before moving onto the next world in Super Mario Galaxy 2. I'm up to World 4, but in Boulder Bowl Galaxy on World 2 there is a hidden star that is still eluding me.

What actions do I need to do to locate this star? If its availability is caused by an in game event what do I need to do to trigger the in game event?

Solution 1:

There's a star you can only earn after getting a message at the Mailtoad from one of the robots with the gear heads. After that you'll be able to meet him in the galaxy, he'll want you to lure a Gooma over to him. So, I guess keep playing and come back later.