Population II

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. Often now simply abbreviated as either Pop I, II or III, these differences were later shown to be highly significant, dividing stars into classes by their chemical composition or metallicity, whose small proportion of stellar matter comprises of the "heavier chemical elements" beyond the more common elements of 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.

Population I stars are young stars with high metallicity, while Population II stars are older stars with low metallicity.

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).

Current theoretical stellar models show that most high-mass Population III stars quickly exhausted their fuel and exploded in extremely energetic pair-instability supernovae. Those explosions would have thoroughly dispersed their material, ejecting metals into the interstellar medium (ISM), to be incorporated into the later generations of stars. Their destruction suggests that no galactic high-mass Population III stars should be observable. However, some Population III stars might be seen in high-redshift galaxies whose light originated during the earlier history of the universe. None have been discovered. Stars too massive to produce pair-instability supernovae would have collapsed into black holes through a process known as photodisintegration, but some matter may have escaped during this process in the form of relativistic jets, and this could have "sprayed" the first metals into the universe.

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