| Population II |
|
A collective name for stars and star clusters within the Galaxy that exhibit characteristics suggesting they are relatively old and occupy a spherical halo around the galactic centre, rather than being confined to the galactic plane. Population II stars typically contain significantly less of the elements heavier than helium than do other stars, have large space velocities relative to the Sun and other stars in the galactic disc, and have orbits in the Galaxy that are very elliptical and highly inclined to the galactic plane. Globular clusters in particular belong to Population II. The characteristics are explained if the Population II stars formed before the Galaxy collapsed to its present flat structure and before the interstellar medium was enriched with heavier elements through mass loss from evolved stars. See also: Population I. |