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
For the time being, ignore the Hungry Luma you meet on this level.

Boss fight. Jump into a 'sling pod' (a sticky part of the web), pull yourself back using the pointer, and launch yourself at the Tarantox's backside. Tarantox will turn upside down, and you should then launch yourself at the three weak points on its underbelly. Repeat.

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 than during Star 1. The time limit shouldn't be a problem on this level, but be careful not to die, as there are no mid-level checkpoints.

Star 5 (Purple Comet): Purple Coin Spacewalk
Available when a Purple Comet is in this Galaxy. Collect 100 Purple Coins within a 2 minute time limit.

Star 6 (Hidden): Yoshi's Unexpected Appearance
Enter Star 3. Proceed through the level as for Star 3. When you meet the Hungry Luma, feed it 50 Star Bits to create a new planet in the shape of Yoshi's head. Default all the Goombas on this planet to gain the Star.