Population II

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Populations I and II

Population II

astronomy

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July 23, 2026

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Populations I and II — Populations I and II Population II stars in the globular cluster M80 in an image taken by the Hubble Space Telescope.

In astronomy, Population II refers to the oldest observed stars and star clusters, formed approximately 10 to 13 billion years ago. These stars are rich in hydrogen and helium but significantly poorer in elements heavier than helium, as these heavier elements had not yet been extensively created at the time of their formation. Population II stars are found in the halos of spiral galaxies and within globular clusters, as well as in elliptical galaxies.12

The concept of stellar populations was introduced by astronomer Walter Baade in the 1950s to categorize stars based on their age, chemical composition, and location within galaxies. Population I stars, in contrast, are younger and contain a higher abundance of heavier elements.12

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Populations I and II
II, in astronomy, two broad classes of stars and stellar assemblages defined in the early 1950s by the German-born astronomer Walter Baade. The members of these stellar populations differ from each other in various ways, most notably in age, chemical composition, and location within galactic…
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II, in astronomy, two broad classes of stars and stellar assemblages defined in the early 1950s by the German-born astronomer Walter Baade. The members of these stellar populations differ from each other in various ways, most notably in age, chemical composition, and location within galactic…
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star cluster
Globular clusters
Globular clusters are composed of Population II objects (i.e., old stars). The brightest stars are the red giants, bright red stars with an absolute magnitude of −2, about 600 times the Sun’s brightness or luminosity. In relatively few globular clusters have stars as intrinsically faint as the Sun been measured,…
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Globular clusters are composed of Population II objects (i.e., old stars). The brightest stars are the red giants, bright red stars with an absolute magnitude of −2, about 600 times the Sun’s brightness or luminosity. In relatively few globular clusters have stars as intrinsically faint as the Sun been measured,…
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Cepheid variable
Population II Cepheids are much older, less luminous, and less massive than their Population I counterparts. They fall into two groups—W Virginis stars with periods greater than about 10 days and BL Herculis stars with periods of a few days.
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Population II Cepheids are much older, less luminous, and less massive than their Population I counterparts. They fall into two groups—W Virginis stars with periods greater than about 10 days and BL Herculis stars with periods of a few days.
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long-period variable star
…larger class of stars called Population II (older stars found mainly in the galactic core and halo). Another group, that of variables with periods of a year or more, mostly belong to Population I (younger stars found generally in the spiral arms of a galaxy).
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…larger class of stars called Population II (older stars found mainly in the galactic core and halo). Another group, that of variables with periods of a year or more, mostly belong to Population I (younger stars found generally in the spiral arms of a galaxy).
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