Bowser Jr.'s Lava Reactor

Bowser Jr.'s Lava Reactor is a galaxy in Super Mario Galaxy. This is where Mario battles King Kaliente once again and receives a Grand Star. The galaxy is one extremely large planet made completely 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 a bullet bill can break.

Lava Reactor
This planet is large and made of lava, there are several sinking platforms on top of it in a circle formation. This planet is home to King Kaliente and is where Mario fights him for the second time.

Enemies

 * Goombas
 * Electrogoombas
 * Bullet Bills
 * 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, 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 platforms sinking platforms. There, Mario will encounter two brown Cosmic Bloopers 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. Upon defeating King Kaliente for the second time, Mario will receive the sixth Grand Star.

Trivia

 * There is a glitch here. 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 and, if done correctly, the Lava Planet's gravity will attract Luigi and he will lose a stock after falling in the lava.
 * This level is the only optional boss battle, as you can get the 60 stars required without this Grand Star.