From the Super Mario Wiki
The Stardust Fields are the border region between the Beanbean Kingdom and the Mushroom Kingdom. The place is a purple field that contains barrels and fallen stars, and there's a perpetual night sky in the fields. The Starshade Bros. occupy the area in some manner, having access to several underground areas with star-shaped entrances. The area is mostly in the Beanbean Kingdom, though a small portion of it (called the Beanbean Borderlands) is in the Mushroom Kingdom. The Beanbean Borderlands is the first place that Mario and Luigi visit when Bowser's Koopa Cruiser crashes. The border between the two kingdoms is guarded by the Border Bros., and to cross the border, travelers must complete a round of jump rope in a minigame called Border Jump. Mario and Luigi can also return to do the Border Jump again in exchange for Hee Beans, though they won't be allowed to cross back to the Mushroom Kingdom side.
Once in the Beanbean Kingdom proper, Mario and Luigi meet the large Tolstar. They had to free Bowser from Tolstar's cannon by giving him 100 Beanbean Coins. To gain more coins, Mario and Luigi go further into the fields, where they also learn how to Spin Jump and High Jump from the Starshade Bros.. After collecting enough money and giving it to Tolstar, he reveals that he had lied about freeing Bowser simply to be greedy, and then attacks them. Once Tolstar loses, Sergeant Starshade comes back out, irritated by the noise of the battle. He light the cannon Bowser is in, launching him away. Afterwards, Mario and Luigi head to Hoohoo Village.
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