The Halo Stars are the ancient formation of stellar bodies that encircle the outer edge of the Milky Way Galaxy, representing the last stellar clusters to be encountered before one enters the eternal, frigid night of the intergalactic void. They are believed to be the oldest stars in the galaxy and their galactic circumference extends for approximately 200,000 light years, usually grouped in a cluster formation. The Halo Stars may also be intermixed into the shroud of dark matter that is known to extend beyond the visible disk of the galaxy for several hundred thousand light years. These stars are older than most can understand, and are characterised by mystery and danger. They have remained largely unexplored by the Imperium of Man and it is unknown what kinds of habitable worlds, if any, may circle them. The Halo Stars have a malignant reputation amongst Imperial starfarers and it is believed by many, including quite a few Rogue Traders and Adeptus Mechanicus Explorators, that those foolish enough to venture out into those mysterious reaches never return. Their beliefs have some credibility, as pirates, Warp entities, and new species of hostile Xenos often dwell in the frontier's darkness. Nonetheless, the braver (or foolish) explorers and entrepreneurs always take the plunge into them for good or ill. The Koronus Expanse is an area of the Halo Stars adjacent to the Calixis Sector in the Segmentum Obscurus. The Imperium has begun exploration efforts into the Expanse, led by the fleets of various Rogue Traders (who were in turn assisted by ancient Explorator star charts from a lost fleet).
- Codex Imperialis (2nd Edition), p. 9
- Warhammer 40,000 Rulebook (5th Edition)
- Rogue Trader Core Rulebook (RPG), p. 306