Super Mario Galaxy/Space Junk Galaxy

Space Junk Galaxy is accessible from the Fountain observation dome. You need 9 Stars to enter.

Star 2: Kamella's Airship Attack
Boss fight.

Star 3: Tarantox's Tangled Web
Boss fight.

Star 4 (Speedy Comet): Pull Star Path Speed Run
Available when a Prankster Comet is in this Galaxy. Do the same as for Star 1, only within a 4 minute time limit. There are more coins to collect on this level.

Star 5: Yoshi's Unexpected Appearance
After you gain Star 3, enter that Star again. Feed 50 Star Bits to the Hungry Luma to create a new planet in the shape of Yoshi's head. Default all the Goombas on this planet to gain the Star.