From the Super Mario Wiki
Meteor Ghosts are ghosts only found in the Observatory in Luigi's Mansion. When Luigi first enters the Observatory, the first thing he notices is a small telescope. Luigi can then walk towards it and get a better look. When he looks through, he notices a small moon, at which point the Meteor Ghosts try to attack. Luigi is able to suck a Meteor 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.