Here Be Dragons
Imperium of Man
The Imperium, denotes the million or so inhabited worlds that mankind has settled across the Milky Way Galaxy. It is divided into five Segmentums: Solar (center), Ultima (east), Tempestus (South), Pacificus (West), Obscurus (North). Each Segementum is further divided into sectors. The Halo Stars are denoted by the ring of stars that exist outside of the realm of man. Often this is marked by fierce Warp storms of chaotic energy that prohibit warp travel through them.
Rogue Trader takes place just outside of the Calixis Sector of the Segmentum Obscurus.