From the Super Mario Wiki
A Mega Star is a Starman-like object found in ? Blocks in the game Super Paper Mario. Upon collecting a Mega Star, the player's character becomes an enlarged 8-bit sprite from the original Super Mario Bros. game (although in the beta version of Super Paper Mario, Mario's sprite was recoloured to resemble his modern appearance). The background music will also instantly change to a remix of the original Starman theme known as "An Unrivaled Battle", according to The InterNed. While the Mega Star is in effect, the player may smash through blocks, pipes, and enemies, similar to the Mega Mushroom in New Super Mario Bros..
In Chapter 3-1, there is an optional segment involving a Koopa Troopa grabbing a Mega Star from a block and becoming its original 8-bit sprite in giant form, known as Mega Koopa. The heroes can find another Mega Star nearby and run into the Koopa to destroy it. Although getting the star isn't necessary, the player can still attack the Mega Koopa just as they would a normal enemy by using the heroes' normal attacks, Pixls, or attack items such as a Fire Burst. Additionally, the segment can only be accessed by entering the area from the correct door.
Mega Mario and Mega Luigi are both short and fast, while Mega Peach and Mega Bowser are both tall and slow. Mega Stars appear in Chapter 1-1, 2-1, 3-1, 4-2, 5-3, and 7-4. Three Mega Stars appear in 3-1.
Mega Stars also appear in the Flipside Arcade game "Forget Me Not". When found after flipping a card, each Mega Star gives the player points and also temporarily flips all the covered cards over to reveal what each card shows.
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