Populations I and II stars are two broad classifications used by astronomers to group stars and stellar assemblages by the abundance of heavy elements (“Star Populations” 2369). The property that is being used to classify star populations is metallicity, which is “the proportion of the stellar mass made up of chemical elements other than hydrogen and helium” (“Star Populations” 2369). According to this classification, star populations with mass fraction of metals or elements that are heavier than hydrogen and helium that is similar to that of the Sun fall into the category of Population I, and those that have mass fractions of heavy elements that are significantly lower than that of the Sun are classified as Population II (“Star Populations” 2369). Star metallicities range from 10 percent to 300 percent of the Sun metallicity for Population I stars and from 0.1 percent to 10 percent of the solar value for Population II stars (COSMOS). Star populations with negligible abundances of metals are referred to as Population III stars. Given that stars’ properties can be placed on a continuum, sometimes it is difficult to classify them according to their metallicities. Therefore, astronomers also classify stellar populations according to their photometric properties.
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It is generally believed that Population III stars were the first group of stellar populations that evolved in the Universe under the condition of only hydrogen and helium being present in the composing gas, which fills interstellar medium (“Star Populations” 2349). The evolution of this medium is triggered by various forces and processes such as supersonic turbulence, the gravitational force, magnetic field, and angular momentum among others. In the framework of the standard cosmological model, “primeval matter emerged from Big Bang as a mixture of H and He with negligible quantities of elements heavier than Li” (Miglio et al. 156).
Taking into consideration the fact that there are no known stars with zero metal content, Population III stars had masses higher than 1M (Miglio et al. 156). It means that in terms of initial mass function, they were substantially different from the present ones. At the time of their explosion as supernovae, they introduced various chemical elements into the medium within galaxies, thereby giving rise to Population II stars. Within the Milky Way galaxy, meta- poor star populations can be mainly found in the halo (“Star Populations” 2369). It should be noted that light metals that dominate the abundance ratios of Population II stars can only be formed in Type II supernovae explosions; therefore, it is believed that they were created in approximately the first billion years of the formation history (COSMOS). If one were to look in the sky today, they would discover that metal-poor group of stars are halo stars and are mainly found in globular clusters and dwarf galaxies (Miglio et al. 156). It should be mentioned that stars are also classified by their location in the galaxy: thin disk, thick disk, halo, and bulge (COSMOS). Population I stars have higher metallicities than that of the Sun and are mainly found within the thin disks of galaxies. However, some Population I stars are also located in the bulge (COSMOS). Unlike Population II stars, which are formed in Type II supernovae explosions, Population I stars were formed in Type Ia supernova explosions of white dwarfs within binary systems.
COSMOS. “Population II.” Astronomy, Web.
Miglio, Andrea, et al. Asteroseismology of Stellar Populations in the Milky Way. Springer, 2014.
“Star Populations.” Encyclopedia of Astrobiology. 2nd ed., 2015.