Fallen Star

Fallen Stars are star-shaped items that fall from the sky during nighttime, brimming with mana, even lighting up as bright as a torch. The player can consume the Fallen Star to regenerate 20 mana, or use 10 to make a Mana Crystal, an item that both restores 20 mana and increases the player's maximum mana by 20 to a maximum of 200. If the player wishes to have mana, they must search for these at nighttime, as they vanish at sunrise (4:30)

Furthermore, they're known to hurt when hit by one. Monsters take extremely heavy damage when hit by a Fallen Star during its fall. A Fallen Star's strike is powerful enough to instantly destroy, or at the very least halve the health of, one of Skeletron's arms, inflicting about 1000 damage with each hit. This does not, however, mean that a player will die if struck by one, as you will in fact catch the star. It's also notable that the damage inflicted by a Fallen Star inspired the creation of the Star Cannon.

The Fallen Star was rumoured to be modelled off of Super Mario's star power-up. It's stated on the owners blog that it's later use may be used to create Super Potions.

Dropping a star during the day will cause it to disappear.