The Empire, formerly the Great Empire of Bira, Sol and Mu Arae, was a galactic sovereign state. It was founded by three sentient species (Biran, Humans and Mu Araen) which, by virtue of their complementary environmental requirements, were able to develop an effective symbiotic relationship and out-compete monospecific galactic civilizations, of which there were a few of note. (Including more xenophobic Human and Biran civilizations.)
The Empire was a constitutional elective monarchy. The Emperor was elected for life by the Senate, the members of which were elected by planetary governments. Emperors were Human or Biran; Mu Araen were too psychologically different to attract the votes of the other species. The Senate also periodically confirmed various cabinet and high military officers appointed by the Emperor.
The Empire was heterogeneous and limited its interference in planetary affairs so long as sentient rights, particularly the rights of visiting imperial citizens, were respected, and peace was maintained. Apart from its symbiotic advantage, its longevity and success can probably be partly attributed to this "hands off" nature, which lessened the inclination of inhabitants to attempt to break away. Besides convention, the ability of the Emperor to implement specific policies was also limited by the large number of hierarchical levels in the Imperial bureaucracy, which tended to increase local control through implementation decisions.
Ultimately, the downfall of the Empire was a combination of sociological and technological challenges: the appearance of a violent religious movement in possession of bulk antimatter and opposed to the use of wormhole travel, something absolutely fundamental to the existence of the Empire. The Empire had weathered enemies with technological advantages and divisive mass social movements of various kinds before, but was unable to survive them in combination. The Empire collapsed when the Guardian sect destroyed many wormhole gates with antimatter, leading to a dark age in most, possibly all, of its former territory.