stellar populations I and II
/ POP-yoo-LAY-shuns /
Just as the people in an old city can be sorted into newcomers and long-settled families, the stars of a galaxy fall into broad generations distinguished by their chemistry and age. Astronomers call these stellar populations. The two classic groups, named simply Population I and Population II, are the galaxy's young, metal-rich stars and its old, metal-poor stars.
Population I stars are the younger generation: they have relatively high metallicity (plenty of elements heavier than helium), because they formed from gas already enriched by earlier stars. They live in the flat, gas-rich disk of the galaxy, move in orderly circular orbits, and include the Sun. Population II stars are the elders: metal-poor, often very old, and found in the galaxy's puffed-up halo and in globular clusters, swooping along steep, plunging orbits. The names are a historical accident from when Walter Baade first split the two groups in 1944 — confusingly, Population I (the higher number) is the younger, more metal-rich group.
Stellar populations matter because they are a fossil record of how a galaxy was built. The metal-poor Population II stars are relics of the galaxy's earliest days, when little had yet been forged in stellar furnaces; the metal-rich Population I stars are the recent products of a chemically mature galaxy. Reading the populations of a galaxy lets astronomers reconstruct its history of star formation and chemical enrichment. A hypothetical first generation of all — the pristine, metal-free Population III stars — is thought to have existed but has never been directly observed.
The Sun, in the galactic disk and rich in metals, is a Population I star. The ancient stars crowded into a globular cluster like M13 are Population II — metal-poor survivors from when the Milky Way was young.
Young and metal-rich versus old and metal-poor: two generations in one galaxy.
The numbering is backwards from intuition: Population I is the younger, metal-rich group, and Population II the older, metal-poor one. The truly first stars, metal-free Population III, are predicted by theory but have never been directly seen.