In super Mario galaxy how do you get the mystery power star for dreadnought galaxy?

1.go to dreadnought galaxy

2.if you have solved all three main power star levels over 1 star a question mark

will be there click on that level

3.when you get to the part with the colorful planet on 1 corner a hungry luma

will be there (I'm not sure how many star bits it takes so get all you can)

4.when you give all the star bits to hungry luma go into the launch star

5.it will take you to a level where you have to explode garbage (just like a another

galaxy)

so i hope that helped and i hope you can beat the game and have fun