Super Mario Galaxy/Hurry-Scurry Galaxy

Hurry-Scurry Galaxy is accessible from the Fountain observation dome. You need 18 Stars to enter.

Shrinking Satellite
Run along the collapsing path to get to a launch star. Each tile on the path will shrink gradually, so try to hurry along it, but don't go too fast so that you will fall. After that, take the launch star to get to a satellite with more of the shrinking platforms. Collect all of the musical note shaped items here. To do this, go along in a horizontal path all the way around the planet, then go in a vertical path. After that, the black hole you will fall into if you fall off of the planet will become a power star. Deliberately fall off of the planet to get your power Star!