Stars observed in our galaxy appear to group into two general types called Population I and Population II. (A hypothetical third group, Population III, does not occur in our galaxy.) The criteria for classification include age, chemical composition, location in the galaxy, space velocity.

Mostly because of age, the different populations are distributed differently on the Hertzsprung-Russell diagram. This can be helpful to classify entire groups of stars in those cases where all the stars are believed to share a common origin.

In general, population number (I, II, III) increases not with the generation of a star, but instead with its age.

Young Stars

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Population I stars contain significant amounts of elements heavier than helium (termed "metals" by astronomers). These heavy elements were produced by earlier generations of stars and spread by supernova explosions, or the dispersion of material from planetary nebula. Our own Sun is a Population I star. They are common in the spiral arms of the Milky Way galaxy.

Old Stars

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Population II stars are the first long-lived stars to have formed after the Big Bang, and therefore have low metal abundance. As a result, they are unlikely to have much in the way of planetary matter orbiting them. Population II stars are found in globular clusters and in the Galactic Halo of the Milky Way (as CS22892-052, CS31082-001, HE0107-5240, HE1327-2326).

Population II stars are much older than Population I stars, contrary to what one might expect from the numbers assigned to them. This is a result of historical holdover: when the compositions of stars were first being surveyed, it was not known why some stars were more metal-poor than others.

Oldest Stars

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A hypothetical third population of stars are Population III stars, which have not yet been observed. These are presumed to be stars of zero metallicity that accounts for heavy elements in quasars and the period of reionization shortly after the Big Bang. Although predicted, none have been observed, nor is there yet sufficient indirect evidence to confirm their existence. They are predicted to have been extremely large, hot, and short-lived, possibly with masses several hundred times that of the Sun.

See also

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• Abundance of the chemical elements
• Bulge (astronomy)
• Metallicity
  • Metal-rich or Population I stars
  • Metal-poor or Population II stars
  • Metal-less or Population III stars

Physical cosmology | Stars | Stars by metallicity | Star types

Població estelar | Population (Astronomie) | Population stellaire | Popolazioni stellari | Sterrenstelsel | Звёздное население | Hviezdna populácia