- For other uses, see Xi.
The Xi Cluster (or the Xi Star Cluster) is a region of space located somewhere in the Alpha or Beta Quadrants, near to the borders of the space territories of the Federation and the Romulan Star Empire, and somewhat proximate to the Klingon Empire. The Xi Cluster was possessed of unique stellar phenomena, particularly in the Tomariian system, a trinary group dominated by the red giant IIlan. Because of the close interaction of IIlan and it's closest secondary star, there exists only a narrow corridor of travel for starships to approach the planets in that system, which are tidally locked on opposing sides of their orbits and shielded by magnetic fields.
The Xi Cluster was the center of the Tomariian Empire in the 23rd century. In 2270, the Tomariian civilization made itself known in a series of strikes against the major powers of local space, causing a number of Starfleet, Klingon and Romulan expeditions to their homeworld, Tomarii. (TOS novel: Black Fire)