Bonus stars in Super Mario Galaxy?

The bonus stars are stars that they don't tell you how to find, you have to figure it out. To do the bonus star that you mentioned, simply go the the star with the question mark, don't get off of the original plannet, kill all of the charactors and you will see a rock that is sparkeling. Spin into it and it will turn into a star that will get you to another plannet. Once you do that the rest is pretty self explainitory. Hard star to get though, did it 3 times before I got it.