The phrase "halo stars" refers to stars that are found in the outer regions of a galaxy, away from the main central disk. They have a faint glow around them that forms a halo-like shape. Full definition
The odd motion marks them as members of the Milky Way's ancient population of halo stars. (newscientist.com)
Extremely scarce in elements heavier than hydrogen and helium («metals»), HE 0107 - 5240 was found as part of a search for metal - poor halo stars in the Hamburg / ESO Survey, which gave it its «HE» designation in combination with its position (see: press releases from ESO and the University of Michigan; and Christlieb et al, 2002). (members.nova.org)
Like other halo stars, it has little iron, because it arose before most of the stellar explosions that spewed the element into space. (sciencemag.org)