星族I和星族II(Populations I and II)
两类大型的星族，其成员在年龄、化学组成和在银河系中的位置等许多方面互不相同。这一概念由巴德（W. Baade, 1893～1960）提出。星族I由年轻的恒星和星群组成，存在於在银河系和其他旋涡星系的旋臂及其附近和某些年轻的不规则星系（如麦哲伦星云）中。星族I中的成员被认为是源自星际物质的不同变化过程，包括超新星的爆炸，并因此使其组成物质带有偏重的元素。星族II由较老（通常在10至15亿年）的恒星和星群组成，被认为是源自在银河系早期形成的星际物质的，主要由氢和氦构成，存在於星系晕、星系和球状星团中，在椭圆星系中也大量存在。天文学家们有时用星族III来指在大爆炸後出现的第一代恒星。
English version：
Populations I and II
Two broad classes of stars and stellar groupings, whose members differ primarily in age, chemical composition, and location in galaxies. It was distinguished and named by Walter Baade (1893-1960). Population I consists of younger stars, clusters, and associations. These occur in and near the arms of the Milky Way system and other spiral galaxies and have been detected in irregular galaxies (e.g., the Magellanic Clouds). Population I objects are thought to have originated from interstellar gas that has undergone various processes, including supernova explosions, which enriched their constituent matter with heavier elements. Population II consists of older (generally 1 billion-15 billion years old) stars and clusters, presumably formed from interstellar gas clouds that emerged very early in a galaxy's history. Consisting almost entirely of hydrogen and helium, they are found in the galactic halos of spiral galaxies, in globular clusters, and, in large numbers, in elliptical galaxies. Astronomers sometimes refer to a Population III as the very first generation of stars to emerge after the big bang.