什么是 Stellar population？

A stellar population is a group of stars of a galaxy that share a range of ages, of chemical compositions and of orbits. The young stars of the thin disc, rich in elements heavier than helium and on nearly circular orbits, are traditionally called population I; the old stars of the halo and of the globular clusters, poor in heavy elements and on orbits of large random motions with little net rotation, population II.