Shining Ghost

Shining Ghosts are ghosts only found in the Observatory in Luigi's Mansion. They rain down on Luigi once he looks through the Observatory's telescope and sees the moon. Luigi is able to suck a Shining Ghost into the slot of his Poltergust 3000. He can then fire it at the moon, causing the moon to explode, revealing Mario's Star.

Trivia

 * As shown in the E3 2001 trailer, the Shining Ghost originally had a different behavior: it was seen bouncing around Luigi in the Nursery, and could be sucked up by the Poltergust 3000. It was also completely colored white in this version of the game.

Reference
Spettro cadente