Celestial Spirit

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 rarer 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 named after a different astronomical constellation.

Contracts and Rules
When a Celestial Spirit mage receives a Key and opens its Celestial Gate for the first time, they have to set up a contract with the respective spirit. This contract consists of asking the spirit which days it can be summoned by the mage. This simple agreement, much to the shock of Natsu and Happy, forms a dedicated, serious bond between the Spirit and summoner.However, under certain circumstances, i.e. during battle, a contract can be asked to be completed later. Contracts can be broken if the mage gets arrested, releases the spirit by him or herself, or dies. When a Celestial Spirit mage summons a spirit, it appears directly next to them, as that's where the gates to the Spirit world had been opened. It's impossible to summon spirits elsewhere.

The spirits must also abide by certain rules enforced by the Celestial Spirit King. The only rule that has been introduced in Fairy Tail so far is that a spirit may not directly or indirectly kill its summoner under any circumstances. When the gates are closed, it is required from both the summoner and the spirit to agree upon the gate's closure. However, spirits can also be "forced closed" in battle, which means they can be forced back into the Celestial World by the summoner's will, even though not every Celestial Spirit mage is capable of doing that. Celestial Spirits also have different categories of sorts, that fall under the basic summoning conditions from their Owner. For example, Loke summons himself most of the time without Lucy doing so herself, Virgo also has summoned herself once, as has gemini and If trust between the Celestial Spirit and the Owner is not an issue, contracts can be temporarily dissolved. It also seems that if the Celestial Spirit is strong enough, they can use their own magic to stay in the Human World, when/if the contract is not in action. The strength of the Celestial Spirits is affected by the magical strength of their summoner - Lucy once said that her spirits could become even stronger than Natsu if she trained more.

Types and Abilities
Gold Keys
 * Aquarius: Summons the Water Bearer, Aquarius.
 * Taurus: Summons the Bull, Taurus.
 * Cancer: Summons the Crab, Cancer.
 * Virgo: Summons the Maiden, Virgo.
 * Sagittarius: Summons the Centaur, Sagittarius.
 * Leo: Summons the Lion, Leo. (Temporarily Nulled)
 * Gemini: Summons the Twins, Gemi and Mini.
 * Aries: Summons the Ram, Aries.
 * Scorpio: Summons the Scorpion, Scorpio.
 * Capricorn : Summons the Goat, Capricorn.

Silver Keys
 * Crux: Summons the Southern Cross, Crux.
 * Horologium: Summons the Clock, Horologium.
 * Lyra: Summons the Lyre, Lyra.
 * Nikora: Summons the Canis Minor, Plue.
 * Caelum: Summons the Chisel,Caelum

Celestial World
All Celestial Spirits spend their time in the Celestial Spirit World when not called through by their summoners. Being in the World seems to have some sort of rejuvenating effect on the spirits, restoring their health after being in the human world which, over long periods of time, can potentially kill the spirit. The reverse is also possible: humans are unable to breathe in their World so after a period of time, albeit a much shorter one, they would die. Such an occurrence (a human entering the Celestial World) is a serious breach of contract. This rule, however, is not absolute, since Natsu survives an extremely brief period in the Celestial Spirit World when he catches hold of Virgo just as she transits through the spirit world to reappear elsewhere in the human world at Duke Everlue's command.

Celestial Spirit King (星霊王 Seirei-ō)

 * Summoner: (None)

Who rules all of the spirit world and enforces the rules among all the spirits. He is the most powerful of all the spirits although it is unknown if he gives out his key to humans or even if he has one. He has the power to stop time in the human world, also he can choose what he wants to stop.

''Celestial Spirit King redirects here. For the episode, see Celestial Spirit King (episode)''.

Gold Keys
Spirit mages have the potential to obtain extremely rare Gold Keys, which open the gates of the "Ecliptic Zodiac". According to Lucy, while there are large number of silver keys, there are only 12 gold keys. Each gold key has the Zodiac symbol of the one it is used to summon on it. Lucy posses all of the keys except of the remaining Ecliptic Zodiac Spirits; Libra, and Pisces.

Aquarius
"The Water Bearer" Aquarius is in the form of a mermaid carrying a jug of water. Even getting Loke's key, Lucy continues using and considering Aquarius as her best spirit and her trump card, althought she is the hardest to control. When she attacks both Lucy and her opponent are carried away with water. She's also dating the Celestial Spirit Scorpio. Lucy may only summon Aquarius while her key is in the presence of water.
 * Summoner: Lucy Heartfilia

Taurus
"The Golden Bull" Taurus is a minotaur with black and white splotched skin, with a nose ring and a black speedo with a belt. Taurus' preferred weapon is a giant two-bladed axe, which with Taurus' gargantuan strength becomes a deadly weapon. He is also quite a pervert.
 * Summoner: Lucy Heartfilia

Cancer
"The Great Crab" Cancer has a weird crab-claw hairstyle with six crab legs coming out of his back, he also has black sunglasses. He wields a pair of scissors that he expertly uses for both melee attacks and haircuts. Happy thought he would say "-kani"(Japanese for "crab") at the end of the sentence, while in fact he says "-ebi" (Japanese for "shrimp"). Happy was shocked and even asked Lucy to dismiss Cancer. In the anime, Cancer says "-choki" (Japanese for snip or cut) in the presence of Erza; apparently, he is infatuated with her and when Erza suggested he say "-choki" instead of "-ebi" he abides without hesitation.
 * Summoner: Lucy Heartfilia

Virgo
"The Maiden" Virgo has a normal black and white maid outfit, with chained shackles around her wrist. She has the ability to dig holes underground. She is extremely obedient and loyal to whoever her master may be; she will even change her appearance to her master's liking. She also asks her master if they will punish her after she has helped them, even if she has done a great job, indicating she may enjoy it. She has two forms, her "Gorilla" form and her "Maid" form.
 * Summoner: Lucy Heartfilia, Duke Everlue (formerly)

Sagittarius
"The Archer" Sagittarius is a man wearing a horse costume much to Lucy's surprise with a great skill in archery, as setting a machine on flame with arrows is child's play to him. He has many different speech patterns as he tends to say "hello hello" ("Moshi moshi") at the beginning or the end of the sentence and adds "-dono" at the end of Lucy's name.
 * Summoner: Lucy Heartfilia

Leo
Leo "The Lion" appears as a man in a designer suit with his hair in the form of a lion's mane, and is claimed to be one of (along with Aquarius) the most powerful key of them all, as he is the leader of the Zodiac. Leo the Lion is later found to be Loke, and joined Lucy after she helped him return to the spirit world. Loke seems to be able to sense when Lucy is in danger and come to her rescue even without the keys or being called. Leo uses Regulus, a form of light magic which draws upon the power of the main star of constellation, Regulus (lat.: Little King, aka Heart/Paw of the Lion). Loke is also able to use Regulus, but because of his weakened state, he could only channel it through his magic quartz ring. Leo also told Virgo to call him big brother. He seems to have an attraction towards Lucy. In the S-class arc he teams up with Gray Fullbuster.
 * Summoner: Lucy Heartfilia, Karen Lilica (formerly)

Aries
"The Ram" Aries is a timid female. She has horns at the top of her head much like a ram. She has white and fluffy clothes, like a sheep. Her magic deals with using wool attacks to reassure her opponents in combat. She is good friends with Loke, who risked his life trying to free her from her former summoner Karen's treatment.
 * Summoner: Lucy Heartfilia, Angel (formerly), Karen Lilica (formerly)

Scorpio
"The Scorpion" Scorpio is a rough man with a large gun for a tail. He is Aquarius's boyfriend and apparently the two have a happy relationship with each other. Scorpio uses a sand based magic, which he blasts a great amount of sand from his tail. Scorpio strangely refers himself as plural, saying "we" instead of "I" in conversation. The reason being that he views himself as royalty and thus uses the "Royal we" (using we instead of I when referring to oneself to demonstrate that one's own people automatically support any decision that their king makes)
 * Summoner:Lucy Heartfilia, Angel (formerly)

Gemini
"The Twins" Two little spirit twins named "Gemi" and "Mini" that can transform into anyone perfectly and replicate both their thoughts and magic. They were formerly Angel's most used spirit but left her when they recognized Lucy's love for Celestial Spirits. Gemini can turn into whoever they touch and during this time they can also know everything that person is thinking and also their abilities. However they can morph for the maximuim time of 5 minutes and also they can only copy 2 people at a time if proceeded they will forget the other one.
 * Summoner: Lucy Heartfilia, Angel (formerly)

Capricorn
"The Goat" is a humonoid goat who uses a lost magic called Human Subordination Magic. He originally had a contract with Layla Heartfilia, but apparantely something happened and he disappeared from the Celestial World for 17 years. He is now a member of Grimiore Heart and one of the Seven Kin of Purgatory.
 * Summoner: Currently none, Layla Heartfilia (formerly)

Silver Keys
They are the very common kinds of Gate Keys. As it was said magic is bought and sold everyday, Silver Gate Keys are one of the magics sold everyday....And they cost alot of money

Crux
"The Southern Cross" Crux is a human like cross, with a mustache and cross shaped nose. Crux has a lot of information about all Celestial Spirits and their masters, either in the present or in the past. He looks like he is sleeping when searching for the information, which confuses other people beside Lucy (like Happy) with whether he is actually sleeping or otherwise.
 * Summoner: Lucy Heartfilia

Horologium
"The Pendulum Clock" Horologium is a large grandfather clock. Horologium has combat and non-combat uses. Examples of non-combat uses are shelter from snow and other climates or transport, while combat examples of Horologium would be defense hardening by hiding inside of him. Lucy always uses Horologium for shelter and when she talks, Horologium would repeat her words then end with "that's what she said" after each sentence.
 * Summoner: Lucy Heartfilia

Lyra
"The Harp" Lyra wears a bonnet and dress. She also has wings on her back, she has the ability to sing beautiful songs, she mostly sings about emotions, but she does take requests. Lucy mostly uses her for musical theater's or competitions. Lyra can only be summond three times a month, but anytime.
 * Summoner: Lucy Heartfilia

Nikoran
"The Canis Minor" Nikoran (or Plue, which Lucy named) is a cute, yet small creature. Nikoran takes up hardly any magic power when summoned, stated by Lucy. Nikoran has no relation with a dog even though he is the "Lesser Dog, Canis Minor".
 * Summoner: Lucy Heartfilia

Caelum
"The Chisel", it appears as more of an item than a being which takes three different forms to attack. So far this is the only Silver Key shown to be used in combat. It is unknown what happened to it once Angel was defeated, but has most likely ceased its contract like the others, yet did not join Lucy.
 * Summoner: Angel (formerly)

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.
 * So far, Caelum is the only Celestial Spirit of both Gold and Silver Keys that doesn't eventually come into Lucy's possession that has appeared in the series.
 * During the Tower of Paradise arc, 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.
 * So far of the Zodiac Keys, Lucy owns 9 out of 12 golden keys.
 * When Lucy was with Horologium in the alternate space, the area they were in was not the Celestial World, but apparently a different space entirely.
 * Another one of the 88 constellations, Eridanus the River, has been the first constellation shown to not be a Celestial Spirit. Instead, it is a extending whip called Fleuve d'étoiles (French for "River of Stars"), that was given by Virgo to Lucy, as a gift from the Celestial World.
 * Ironically, despite being refered to as being extremely rare, there are more known golden keys in the series than silver keys.
 * At least for now, in each elemental category of gold keys, exist a weapon-bearer, a magic owner and a girl.