Bowser Jr.'s Robot Reactor is accessible from the Terrace observation dome. You need 8 Stars to enter.
This galaxy consists of a couple of small round planets.
You start on the outside of a small hollow planet, populated with helmeted Goombas and Bullet Bills. You need to use the Bullet Bills to break the glass cage holding the Launch Star. Use the Launch Star to reach the second planet.
Bowser Jr. will appear and set Megaleg onto you, which is a large robot bigger than the planet itself.
Boss Fight: Megaleg
- Stomp: Megaleg brings it's foot up and presses it down somewhere else. This isn't aimed at you, but, like being crushed any other way, you instantly lose a life. Remember that.
- Bullet Bills: Megaleg has cannons of Bullet Bills on it's legs and head. On each leg he has three cannons together and on it's head there are cannons in a circle. Use some jumping for the legs and use them on the head.
At the start of the fight Megaleg uses Stomp, but don't worry about this as you can easily avoid it. Start climbing the leg and soon you will reach the three Bullet Bill cannons. A trick is to go between two of the cannons and jump when the Bullet Bills come close so the two smash into each other and jump up onto the platform and go as far as you can till you reach the cogs, and the third Bullet Bill will probably smash straight into the platform (as they like to go in a straight line). Jump-spin over the gears (or if your more professional you can long jump then spin. When you get to the head lure a Bullet Bill to the cage on the head (take it slowly) then lure two to smash the metal the the glass and the star's yours.