From Halopedia, the Halo wiki
The Orion complex is a large nebular complex in the Orion Arm of the Milky Way Galaxy. The nebula was once the nexus of Forerunner activity as the core of their ecumene. It was home to the Forerunner homeworld Ghibalb, located at the outer edges of the nebula, the triple star system housing the homeworld of Bornstellar Makes Eternal Lasting and his family, and the Capital, the seat of the Ecumene Council. The cloud itself is situated between 1,500 and 1,600 light-years away from the Sol system and is hundreds of light-years across. The complex contains several smaller nebulae, and many protoplanetary discs and young stars.
Early on in Forerunner history, their nascent civilization settled twelve systems within the complex. Due to a series of novas triggered by early Forerunner experiments with stellar engineering near their homeworld, the entire Orion complex was brightened for the next fifty thousand years. The Forerunners relied on a defensive boundary known as the Maginot Line to keep the systems of the Orion complex safe from enemy assault. During the human-Forerunner wars, human forces never came closer than 15,000 lightyears of the Orion complex.
List of appearances