A star moving faster than 65 km/s relative to the average motion of the stars in the Sun’s neighbourhood (the local standard of rest). High-velocity stars are members of the galactic halo, moving in highly elliptical orbits around the galactic centre. Their high relative velocities result from the fact that they are passing through the galactic disk and do not share the rotation of the Sun and its neighbouring stars around the galactic centre. Such stars may have formed early in the Galaxy’s history, or they may be remnants of smaller galaxies that have merged with ours.