PopulationIISterne

Population II stars, often referred to in German literature as Population II Sterne, are metal-poor stars that formed early in the history of the Milky Way and other galaxies. The designation Pop II was introduced by Walter Baade in the 1940s to distinguish older, halo and bulge populations from the younger, metal-rich Population I stars in the disk. Population III stars, by contrast, are hypothesized to be the first generation with essentially zero metal content; Pop II stars formed from gas that had already been enriched by the first supernovae.

Characteristics: Population II stars typically have low metallicities, with iron abundances [Fe/H] commonly between about −1.0 and −2.5, and some observed stars even lower. They are generally old, with ages exceeding 10 billion years, and constitute a major fraction of stars in the Galactic halo and in globular clusters. They often display enhanced alpha-element abundances, reflecting enrichment by core-collapse supernovae in the early Universe.

Distribution and significance: Pop II stars are found in the Galactic halo and bulge, as well as in globular clusters and in dwarf spheroidal galaxies around large galaxies. Their long lifetimes allow many low-mass Pop II stars to survive to the present day, providing information about early star formation, chemical evolution, and the assembly history of galaxies.

Observational notes: Pop II stars span a range of spectral types, including red giants, subgiants, and low-mass main-sequence stars. Extremely metal-poor Pop II stars with [Fe/H] < −3.0 offer clues about the yields of the first generations of supernovae and the conditions of the early interstellar medium.