Agena (also known as Beta Centauri) is a multiple star system, located about 160 parsecs (530 light years) from Earth, where it is visible as the second brightest system in the constellation of Centaurus. (TOS comic: "The Expansionist Syndrome") The Federation starship USS Agena was named for this system when it was authorized for construction. (ST reference: Star Fleet Technical Manual) Since this is a "real" star system, we know details about its location and makeup not revealed in its Star Trek mentions, where it was only mentioned by name.