Starship Mario is a planet in Super Mario Galaxy 2 used to travel across the universe. It is in the shape of Mario's head. This seems to have replaced the Comet Observatory. Here Mario can select galaxies to travel to.
The way you travel to galaxys is you go to the top of the head and jump on this sort of pedal.Then you select a world and select a galaxy.For example:I would go to world 3 and go to tall trunk galaxy it's as simple as that.You may also see that on the starship marios nose there is an yoshi egg open it up and get a ride!!!!!!.