Celestial Spirit Magic

Celestial Spirit Magic is a type of magic in which Celestial Spirit Mages, summon Celestial 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.

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/her self, 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, 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.

Spirits
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 Archer, Sagittarius.
 * Leo: Summons the Lion, Leo.
 * 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.