星族是银河系中年龄、化学物质组成、空间分布与运动特性较接近的恒星集合，于1927年由布鲁根克特（P. Bruggencate），1944年由美国天文学家沃尔特·巴德区分成现在的三族恒星。

观察银河系内的恒星，可以将她们分为第一星族和第二星族两大类（在理论上还有第三星族，但在银河系内未曾发现）。做为分类标准的是年龄、化学成分、在星系内的位置、和空间速度。

主要的原因是年龄，不同的星族在赫罗图上分布的位置不一样，这就像应用在星团时一样，在星团中，所有的成员被认为有着相同的来源。

通常，区分族群的数字（一、二、三）的增加并不意味着世代交替，只区分彼此间的年龄。

In 1944, Walter Baade categorized groups of stars within the Milky Way from their spectra. Two main divisions were defined as population I and II, with another division known as population III added in 1978. Later these differences were shown to be significant, dividing them by their chemical composition or metallicity, the proportion of stellar matter made up of the "heavier chemical elements", beyond hydrogen and helium. By coincidence, each population group definition has decreasing metal content and increasing age. Hence, the first stars in the universe (low metal content) were deemed population III, and recent stars (high metallicity) are population I.