The Star Imperium is the predecessor government of the current United Planets.
The government of the Star Imperium, based on Alfheim was composed of the Imperial Sovereign, two Imperial Viceroys, the Imperial Parliament, the Prime Minister, the Council of Ministers, and the Council of Twelve.
Over the millennium and centuries, the Imperium was often plagued by the office of Imperial Sovereign becoming vacant since the ruler lacked a child to inherit the office, and the Parliament had to elect a new Sovereign. The Parliament was divided into several factions, so it could be centuries before a new Sovereign could be elected by an absolute Parliament majority, so in practice the Council of Twelve often exercised the powers of regency when the office of Imperial Sovereign was vacant. In cases the office of Sovereign was vacant, various Imperial Viceroys, especially those with the position of Prime Minister, assumed de facto leadership of the Imperium, and gained the informal title of "Quasi-Sovereign". Historians and political scientists during the time of the United Planets often referred to the Imperium as a De Facto Intergalactic Republic.
Moreover, there was demand from the various populations for a more democratic government, and hense this created the Sector assemblys and subsector assemblies, what would become the United Planets Assembly, and finally the transformation of the Imperium into the United Planets with the Imperial Parliament's lawmaking powers transferred to the United Planets Assembly, the offices of Imperial Sovereign and Imperial Viceroy abolished completely, and a parliamentary system where the Prime Minister and Council of Ministers are elected by the United Planets Assembly.