This was in Mario Galaxy II, not the first game. The were no green stars in the sequel. It was a long time ago and I can't really remember exact details about it, but it was the very very last mission in the game; once you'd beaten it, you won and Princess Peach gave Mario his well earned thank you bj. Edit: found a video. IGN makes it look way easy, but trust me when I say it was difficult. I'd beaten every single mission before this and none came close to how challenging the last star was to obtain Edit II: also, Luigi was king. Who the hell played as Mario?