stellar populations

(astronomy) Either of two classes of stars, termed population I and population II; population I are relatively young stars, found in the arms of spiral galaxies, especially the blue stars of high luminosity; population II stars are the much older, more evolved stars of lower metallic content; many high luminosity red giants and many variable stars are members of population II.

One of the categories into which stars may be classified, based on their place in the evolution of the galaxy containing them. The stellar component of the Milky Way Galaxy consists of three populations: the thin disk, the thick disk, and the halo.

The thin disk, originally referred to as population I, is the youngest component of the galactic stellar population. Still actively forming massive stars from molecular clouds, it is confined to within about 0.35 kiloparsec of the plane. (1 kpc equals 3300 light-years or 3.1 × 10¹⁶ km or 1.9 × 10¹⁶ mi.) All of the stars have metallicities lying between about one-fifth and twice the solar value, and star formation appears to have remained constant in this population for about the past 8 × 10⁹ years. One reason for the relatively small thickness of the disk is the low velocity dispersion of the component stars; their motion is completely dominated by the differential rotation of the disk. These stars are found associated with H II regions and OB associations as well as open clusters. See also Interstellar matter; Molecular cloud; Supernova.

The thick disk is an older population, approximately 9–10 × 10⁹ years, roughly corresponding to the range between what was once called population II and population I. Its metallicity lies between about one-tenth and one-third of the solar value. The stars in this population are distributed over greater distances from the plane, up to 1.5 kpc, and have correspondingly larger velocity dispersion. This population also includes globular clusters and subdwarfs that overlap at the lowest end of the abundances with the properties of the halo globulars, although the system of old disk globulars is distributed differently than those of the halo.

Lying around the disk and the nuclear spheroidal bulge, there is a halo, roughly corresponding to the original population II, that extends to considerable distances from the plane, some as distant as 30 kpc. This population has an age of order 10–15 × 10⁹ years. The stars in this region have very large velocity dispersions and do not appear to participate in the differential rotation as much as other stars. Their metallicities are all lower than about one-twentieth that of the Sun and may extend down to 10⁻³ of the solar value. The most metal-poor globular clusters belong to this population. This stellar halo is not the same as the dark matter halo, but is probably embedded within it. See also Milky Way Galaxy; Star.