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.
Planets
Tech Planet
This is where Megaleg is found and battled. It is fairly small and orange, 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.
New Mix Planet
This is a very small planet. Mario simply has to get a Bullet Bill to break open a piece of machinery to reveal a Boost Star, and then use that Boost Star to reach the Tech Planet.
Levels
Megaleg's Moon
Mario begins this level on the New Mix Planet, which contains a Boost Star in a cage. Lure a Bullet Bill over to the cage to free the Boost Star and reach Tech Planet. Bowser Jr. will appear and summon Megaleg. Check the page on Megaleg for more information on fighting this boss.