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Stellar PopulationStars can be grouped into two general types called Population I and Population II. The criteria for classification include space velocity, location in the galaxy, age, chemical composition, and differences in distribution on the Hertzsprung-Russell diagram. 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. Our own Sun is a Population I star. They are common in the spiral arms of the Milky Way galaxy. 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 core of the Milky Way. 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. A third, as yet hypothetical, population of stars are population III stars. These are presumed to be stars of zero metallicity that accounts for heavy elements in quasars and reionization. Although predicted, they have not been observed sufficiently 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 List of astronomical topics: Abundance of the chemical elements, Bulge, Cepheid variable, Galactic astronomy, Metal-rich, Elliptical galaxy, Metal-poor, W Virginis variable, Spiral galaxy
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