Stellar population

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.

Stellar populations

Observation of the spectra of stars has revealed that the metallicity of older stars have fewer heavy elements compared to the Sun. This immediately suggests that metallicity has evolved through the generations of stars by the process of stellar evolution. In current cosmological models, the matter created in the Big Bang was mostly hydrogen and helium, with only a very tiny fraction of light elements like lithium and beryllium. After this, when the universe cooled sufficiently, the first stars were born as extremely metal-poor Population III stars. Without metals, it is postulated that their stellar masses were hundreds of times that of the Sun. In turn, these massive stars evolved very quickly, and their nucleosynthetic processes quickly created the first 26 elements (up to iron in the periodic table).