population I, population II
population I, population IIThe two classes into which stars and other celestial objects within a galaxy can be divided; the distinction is basically that of age, and the most obvious distinguishing characteristics are the objects' space distribution and chemical content (see metallicity). The classification was first made by W. Baade in 1944. He proposed that population I stars are the young metal-rich highly luminous stars found in spiral arms of galaxies – i.e. strongly concentrated in the galactic plane – and associated with interstellar gas and dust. In contrast population II stars are old red stars found throughout elliptical and lenticular galaxies and in spiral galaxies located in the galactic center, in the surrounding galactic halo, and in a disk coextending with the galactic plane but much thicker.
It is now known that in our Galaxy there is a continuum of populations existing between Baade's two classes (see table). The origin of population types is connected with the dynamic and chemical evolution of the Galaxy. Put very simply the earliest stars, formed from the original hydrogen-helium gas cloud, were halo population II stars followed, after further gas contraction produced a relatively thin extended disk, by intermediate population II stars; contraction of the remaining gas into an even thinner disk produced the disk population, which may be young population II stars or old population I stars. Successive generations of population I stars have subsequently formed from interstellar matter that has been slowly enriched with metals produced by mass loss from stars and by supernovae explosions. The youngest extreme population II stars have 20 to 30 times the metallicity of the oldest halo objects.