Bowser Jr.'s Robot Reactor

How do I unlock Bowser Jr.'s Robot Reactor?
Get 8 Power Stars.

Grand Star: Megaleg's Moon
In the level, you will start at the planet with holes that will send you to a black hole. Go to the Bullet Bill cannon and make the Bullet Bill hit the glass dome covering the Launch Star, which will send you to Megaleg's Moon, starting a cutscene with Bowser Jr. which will activate Megaleg. While he can squish you, go to his legs and jump on them. After that, go to the Bullet Bills and hit the dome, where the Grand Star is. After that, there will be walls which can regenerate after a while, so the best way to do it is to get Bullet Bill, jump at the right time, and go to the dome with the Grand Star. This will cause Megaleg to explode, releasing its Grand Star, allowing you to get it. The Grand Star will activate the Fountain.