Super Mario Galaxy/Deep Dark Galaxy

Deep Dark Galaxy is accessible from the Garden observation dome. You need 46 Stars to enter.

The main planet has a beach and an underwater section past a gate. There are also a couple of smaller planets.

Star 1: The Underground Ghost Ship
First you will need to jump into the cannon and fly to the brown planet (you will need to grab the star that makes you invincible to pass through more easily). Grab the Fire Mario and shoot back to where you started off, then quickly run and put fire on the two fire emblems so the gate will drop. Go down under into the lake. For the time being, ignore the shell you will find underwater, as that will lead to the Hidden Star 6. You will see an arrow made out of Star Bits. Follow it and destroy some boxes to twist the lever. The gate will open. Now swim up and again there will be another lever in the corner: twist it. Swim to the top to find a ship, climb on and you will meet the boss. Defeat her and get the star.

Star 2: Bubble Blastoff
First part: Someone is blocking the cannon. Battle the Boss, then go and blast off to another planet.

Second part: At the last planet, ground pound the melons to get the Power Star.

Star 4 (Daredevil Comet): Ghost Ship Daredevil Run
This Star is the boss fight from Star 1, with just one life point.

Star 5 (Purple Comet): Plunder the Purple Coins
100, no time limit. In ghost ship area. Some in water behind you at start. Some you'll need to crouch jump for - look for shadows. Need to jump out of water for some.

Star 6 (Hidden): Boo in a Box
Enter Star 1. Use shell underwater to destroy mines.