As you progress through the Starfield, you will encounter a number of groups that sort of factions with their own objectives and moral philosophies. The United Colonies or UC, a significant figure in Starfield's history, is one of these groups. Some gamers are wondering how to join them.
You will soon find out that Liber Astra and The United Colonies are two of the major factions in Starfield. The UC Vanguard is the grouping of different colonized systems in the form of a Government which is based in New Atlantis on the planet Jemison in the Alpha Centaur system. Like most factions, it is possible to join UC Vanguard by completing a mission called Supra and Ultra.
To start Supra and Ultra mission in Starfield, you have to go to the Vanguard headquarters which is located at MAST, the building is just above the tram spaceport / station. Once inside, speak to Commander Tuala. It should be noted that you will automatically talk to him during the first main mission for Constellation that you will do with Sarah Morgan. After discussing your desire to join the UC, the commander will invite you to take a short test which, as soon as it is completed, allows you to join this faction.