Stars observed in galaxies were originally divided into two populations by Walter Baade in the 1940s. Although a more refined means of classifying stellar populations has since been established (according to whether they are found in the thin disk, thick disk, halo or bulge of the galaxy), astronomers have continued to coarsely classify stars as either Population I (Pop I) or Population II (Pop II). They have even postulated a third population (Population III; Pop III), though stars of this type have yet to be observed.
The classification system is based on the metal content of the stars (their metallicity, usually given the symbol [Z/H]). Pop II stars are metal-poor, with metallicities ranging from approximately 1/1000th to 1/10th that of the Sun (i.e. from [Z/H]=-3.0 up to [Z/H]=-1.0). This means that the gas from which Pop II stars formed could only have been recycled (incorporated into, and then expelled) from previous generations of stars a few times at most, and that Pop II stars form very early in the star formation history of the galaxy.
Further evidence to support the early origins of Pop II stars comes from their abundance ratios, which show that the lighter elements (e.g. carbon and oxygen) dominate over the heavier elements (e.g. iron, nickel) in the chemical composition of the stars. Since light metals are produced primarily in Type II supernova explosions (the explosions of massive stars which have lifetimes of only a few million years), while the heavier elements can only be produced in Type Ia supernova explosions (the explosion of a much older white dwarf in a binary system), the relative lack of heavy metals in Pop II stars indicates that these stars formed in the first billion years or so of a galaxy’s star formation history.
Population II stars are mainly found in the bulge and halo of galaxies.