The Halo Stars are the ancient stars 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. The Halo Stars are believed to be the oldest stars in the galaxy and their circumference extends for approximately 200,000 light years around the galaxy, 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. The Halo Stars are older than most can understand, and are characterised by mystery and danger. The Halo Stars have remained largely unexplored by the Imperium of Man and it is unknown what kind of habitable worlds, if any, may circle them. The Halo Stars have a very bad 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 the Halo Stars never return. The Koronus Expanse is an area of the Halo Stars adjacent to the Calixis Sector in the Segmentum Obscurus that has begun to be explored by agents of the Imperium, led by the fleets of various Rogue Traders.
- Codex Imperialis (2nd Edition), p. 9
- Warhammer 40,000 Rulebook (5th Edition)
- Rogue Trader Core Rulebook (RPG), p. 306