Bowser Jr.'s Robot Reactor

Bowser Jr.'s Robot Reactor is Bowser Jr.'s first galaxy in the game Super Mario Galaxy. In it, Mario fights Megaleg to earn a Grand Star.

New Mix Planet
This is a very small planet. Mario is required to entice a Bullet Bill to follow him and break open a cage to reveal a Launch Star, and then use that Launch Star to reach the Tech Planet. This metal planet has a black hole that Mario needs to avoid. There are also other cages that can be broken to reveal coins or a 1-Up Mushroom. It is also infested with several Astro Goombas and regular Goombas, which make it slightly harder to traverse the narrow pathways while being pursued by a Bullet Bill.

Tech Moon
This is where Megaleg is found and battled. It is fairly small and orange-brown, with many technology symbols all over it. It has small rectangular areas on it that are transparent and seem to have computer components inside them. Despite Megaleg's probable immense weight, the transparent panels never break if Megaleg stomps on them, all that happens is a cloud of dust rises up.

It rotates the New Mix Planet.

Megaleg's Moon
To defeat Megaleg and win this level, Mario must begin on the New Mix Planet. There are three cages on the planet which must be hit by Bullet Bills and a couple of Astro Goombas. Mario must get a Bullet Bill to the third cage which contains a Launch Star leading to the Tech Planet. On the Tech Planet is where Megaleg is actually battled. Mario starts on the ground and Megaleg blasts Bullet Bills and tries to step on him to defeat him. There are multiple cages on the other side of the planet that a Bullet Bill can break open. If Mario runs up Megaleg's leg when it's on the ground, he will reach an area where three Bullet Bills are blasted at once, dodging those Bullet Bills he must then move on to a turning gear area where he must hop the gears.Finally he will reach Megaleg's head where there will be a cage and a Bullet Bill launcher. Mario must destroy the cage to open up another regenerative cage which he must destroy and a third cage in the middle of that one. Once Mario destroys all the cages, Megaleg will explode and Mario will fall to the ground with the Grand Star.

Enemies

 * Astro Goombas
 * Bullet Bills
 * Megaleg (Boss)

Planets Visited

 * New Mix Planet
 * Tech Planet

Trivia

 * In the beta version of Super Mario Galaxy, the mix planet is grassy and lacks Star Bit rocks.
 * If you go to the top of Megaleg and use the first person view to look at the mix planet, you will see that it looks like it's beta appearance, instead of being metallic.
 * In a trailer for Super Mario Galaxy 2 a Megaleg like Robot appears on a similar planet.
 * Around the square panel on the base of the Tech planet where the glass cages are, the so-called technological symbols are S-Block Tetriminoes in different orientations. This is a reference to Tetris, which is another game franchise owned by Nintendo.