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, and space velocity. The Pleiades, an open cluster of stars in the constellation of Taurus. ... The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia Kuklos; or simply the Galaxy) is a barred spiral galaxy in the Local Group, and has special significance to humanity as the location of the solar system, which is located near the Orion... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light years in diameter and approximately 60 million light years distant. ...

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. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude (CM) diagram) shows the relationship between absolute magnitude, luminosity, classification, and surface temperature of stars. ... Globular Cluster M92 in the Hercules constellation. ...

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

Young Stars

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. To meet Wikipedias quality standards, this article or section may require cleanup. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ... NGC 6543, the Cats Eye Nebula A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. ... The Sun is the star of our solar system. ... The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as the Galaxy), is a barred spiral galaxy of the Local Group. ...

Old Stars

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). Metal-poor is a term that is used to describe the chemical make up of an object. ... According to the Big Bang, the universe emerged from an extremely dense and hot state (bottom). ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ... Spiral galaxies have a typical structure related to their history. ... BPS CS22892-0052 (Snedens Star) is an old Population II star located in a distance of 4. ... BPS CS31082-0001 (Cayrels Star) is an old star located in a distance of 4 kpc in the Galactic Halo. ... HE0107-5240 is a newly discovered giant star roughly 36,000 light years away from Earth. ... HE1327-2326, discovered in 2005, is the star with the lowest known iron abundance to date. ...

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

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. Population III stars are a hypothetical population of extremely massive stars that are believed to have been formed in the early universe. ... In astronomy, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. ... Artists impression of quasar GB1508 A quasar (contraction of QUASi-stellAR radio source) is an astronomical source of electromagnetic energy, including light, which shows a very high redshift. ... In Big Bang cosmology, Reionization is the process that reionized the matter in the universe after the epoch of galaxy formation. ... According to the Big Bang, the universe emerged from an extremely dense and hot state (bottom). ...

See also

• 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