Purple Comet

The Purple Comet is a Prankster Comet in Super Mario Galaxy. The comet only appears after the Player has beaten the game. If the comet appears, the Player will need to collect 100 Purple Coins, to get a star.

There are some timed Purple Comet challenges, which have unique music to indicate this (although the Battlerock Galaxy and Dreadnought Galaxy challenges are not timed but still use this music). Timed challenges usually have 150 purple coins rather than 100. Only 100 are required though.