From the Super Mario Wiki
The Boost Star is a purple Star introduced in New Super Mario Bros. U. Unlike the regular Star, the Boost Star can only be collected in Boost Mode. The Boost Star appears only if the players using the Wii Remotes steps on ten Boost Blocks that still have their symbols on them, thus filling the Star Meter. If the Star Meter on the top of the Boost Mode screen is filled, the Boost Star appears. If a player with the GamePad taps on the Boost Star, the player can for a short time instantly defeat enemies and break blocks by tapping on them with the stylus or the finger (including Koopa Troopas and Thwomps). This effect is similar to the Starman's.
The Boost Star is purple on the GamePad. Onscreen, it can not be activated by any character, and is almost invisible, with a slight blue outline.
Names in Other Languages
||Étoile Coup de pouce
||Boost Star (Coup de pouce literally meaning: "helping hand", "encouragement")