Bowser Jr.'s Lava Reactor

Bowser Jr.'s Lava Reactor is a Bowser Jr. galaxy in Super Mario Galaxy. This is where Mario battles King Kaliente for the second time and receives a Grand Star. The galaxy is one extremely large planet made out of lava with very few rocks.

Starting Planet
This planet is small, made of dirt and is half-gone due to its Black Hole core eating away at its surface. There are several Goombas and one Bullet Bill launcher. There are also several cages that can be broken by a Bullet Bill that contain items such as coins, a 1-Up Mushroom and a Launch star.

Lava Reactor
This planet is large and is made up solely of lava. There are several sinking platforms around its northernmost region in a circle formation, with more sinking platforms in a path-like formation across its southernmost region. This planet is home to King Kaliente and is where Mario fights him for the second time.

Enemies

 * Bullet Bills
 * Octopuses
 * Goombas
 * Lava Bouncers
 * King Kaliente (Boss)

King Kaliente's Spicy Return
Upon entering the galaxy, Mario will land on a small planet with a few Goombas, a Bullet Bill Blaster, a caged 1-Up Mushroom, and a caged Launch Star. Mario will have to lure a Bullet Bill to the cage to smash it open, gaining access to the Launch Star. The Launch Star will shoot Mario to the Lava Reactor's surface, which is entirely lava, except for a few sinking platforms. There, Mario will encounter two brown Octopuses that he will have to defeat to access another Launch Star. From there, Mario will encounter Bowser Jr., who will make him fight a much stronger, dark version of King Kaliente while on the sinking platforms. Upon defeating King Kaliente for the second time, Mario will receive the sixth Grand Star.

Trivia

 * There is a glitch in this galaxy. In the first planet, the player has to make a triple jump while playing as Luigi and land the second jump on top of a cage. Then the player has to Spin in middle of the third jump. If done correctly, the Lava Planet's gravity will attract Luigi and he will lose a life after falling in the lava.
 * This level is the only optional boss battle, as one can get the 60 Power Stars required without this Grand Star.