Celestial Spirit

For the Magic see Celestial Spirit Magic.

Celestial Spirits are Magical beings from their own separate universe, the Celestial Spirit World. Mages, called Celestial Spirit Mages, are able to summon spirits by opening their gates through the use of Keys. These Keys are separated into two classes: the more common Silver Keys and the extremely rare Gold Keys. Keys for Celestial Spirits are counted in Units (collectively without regard of Gold or Silver), and a Celestial Spirit Mage is noted by how many Units they have obtained. All Celestial Spirits are each named after a different astronomical constellation. Out of the 12 Gold keys, 10 of them are owned by Lucy.

Images can be found in Bonus Gallery.

Trivia

 * Nakajima from Rave Master made a brief cameo in episode 18, when Lucy talks about the Celestial Spirit Sagittarius and Natsu imagines a octopus with a Nakajima flower head.
 * Originally, summoning Celestial Spirits were supposed to be by cards. However this idea was eventually scrapped when the author thought that it was too similar to another manga work.
 * During her time in the Tower of Heaven, Lucy managed to summon both Cancer and Aquarius during the same day, which from their contract, Cancer can only be summoned on Tuesday, Thursday, Saturday, and Sunday, while Aquarius can only be summoned on Wednesday. This question was later answered in the Q&A of Volume 16 that the contract is actually constantly changing, and the stronger and trustworthy the bond between the Mage and the Celestial Spirit, the summoner can increase the amount of days that a particular spirit can be summoned.
 * Ironically, despite being refered to as being extremely rare, there are more known golden keys in the series than silver keys.
 * The Golden keys appear to be more stylised in the anime, appearing with handles and teeth to the theme of the summoned spirit, while in the manga they appear as ordinary keys with the Constellation's symbol engraved on the top.