From the Super Mario Wiki
Shining Ghosts (formerly known as Falling Stars) 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.
Luigi's Mansion (Nintendo 3DS)