Bowser Jr.'s Boom Bunker

"My dad's gonna get angry if I let you get another Grand Star... Time to break out my Boomsday Machine! I'll just relax in my comfy cockpit and watch you go BOOM!"

- Bowser Jr.

Bowser Jr.'s Boom Bunker is a boss galaxy in Super Mario Galaxy 2, located in World 5. Here, Mario will fight Bowser Jr.'s latest invention, the Boomsday Machine. The main enemies here are Boomerang Bros. and Space Mines. This is one of two galaxies that features a planet exclusive to a Prankster Comet, the other being the Rolling Masterpiece Galaxy. It is also the only Bowser Jr. level that does not feature Earth orbiting in the background.

Planets
NOTE: Unless otherwise noted, all names are unofficial.

Starting Planet
The starting planet of the galaxy. There is a lot of mud on this planet, as well as a few Fizzlits and a Chance Cube. When Mario lands, various parts of Megahammer that Mario fought and defeated earlier in Bowser Jr.'s Fearsome Fleet are found destroyed, including the robot cockpit. A single Bullet Bill still pops out of one of Megahammer's blasters however. There is a glass cage protecting a Cannon on top of the remains of one of the robot's arms. Mario must lure the Bullet Bill over here and have it destroy the cage to access the cannon and progress to the next planet. The entire Toad Brigade is also found here.

Boomerang Bros. Planet
A castle-like planet inhabited by two Boomerang Bros. There is a large target on one side. If Mario launches himself out of the cannon and hits it in the center, he will get a 1-Up. There are also several poles on this planet, and one of them has a Life Mushroom at the top. After defeating the two Boomerang Bros, a Launch Star leading to the next planet will appear.

Cannon Planet
This small planet has only a Checkpoint Flag and a cannon on top of it. Mario must shoot himself out of the cannon and onto another large target on the bottom of the next planet, but he must make sure not to hit one of the four Space Mines orbiting around the top of the planet. There is also a small electric fence extending partway around the middle of the planet. The Checkpoint Flag is absent in the "Boomsday Machine Daredevil Run" mission, as it is moved to the Dark Matter Planet instead.

Spinning Platforms Planet
After Mario hits the next target, he will find himself on a planet consisting mostly of many different colored platforms spinning around at various speeds with a black hole inside of it. The second set of platforms has Space Mines floating over them and a Comet Medal in between two more Space Mines. There are also a few Cloud Flowers here. Mario must use them to make his way to the top of the planet where he will Ground Pound more stone wheels to make the Launch Star appear. This will lead him to the final planet in the galaxy.

Dark Matter Planet
A large, rectangular-shaped planet with a few small platforms floating in dark matter. Following these, there is also a platform that tilts and moves while carrying Mario. He must move from side to side as the platform tilts back and forth and moves along so as to avoid touching the dark matter. At the end of the planet is a Checkpoint Flag and a Launch Star. This planet only appears in the "Boomsday Machine Daredevil Run" mission, and replaces the Spinning Platforms Planet that appears in the regular level.

Boomsday Machine Planet
A cylindrical planet where Bowser Jr. waits in his latest invention, the Boomsday Machine, to battle Mario. There are also multiple air vents located on the planet, which Mario must use in tandem with the Cloud Flowers found on the surface of the planet to propel himself upward where he must Ground Pound Bowser Jr.'s Boomsday Machine a total of three times to defeat it.

Green Star 1
The player will have to break open the cage in order to get in the cannon on the Megahammer Planet. Using the cannon, the player can reach the Green Star located over the target by firing directly at it.
 * Enemies
 * Fizzlits
 * Bullet Bills
 * Planets Visited
 * Starting Planet

Green Star 2
In order to get the next star, the player must jump in the cannon of the Cannon Planet. Using the cannon and aiming to the far left, the player can collect the Green Star by firing directly at it.
 * Enemies
 * Fizzlits
 * Bullet Bills
 * Boomerang Bros.
 * Space Mines
 * Planets Visited
 * Starting Planet
 * Boomerang Bros. Planet
 * Cannon Planet

Prima Official Game Guide description

 * "While Bowser licks his wounds from your last battle, Bowser Jr. resumes the quest to keep you from scoring Grand Stars. You must fight your way to Bowser Jr.'s last stand and defeat his latest invention, the Boomsday Machine, to earn the Grand Star that opens up the next world."

Trivia

 * The backdrop of this galaxy is extremely similar to the backdrop that is present in Bowser's Galaxy Reactor behind The Pathway, as both feature a large sun that is very close to these areas.

Bowser Jr.s Böllerbunker