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Two broad classifications into which the stars and associated nebulae in our Galaxy and others are divided according to a number of dynamical and composition criteria. They are known as Populations I and II. Population I is essentially the younger generation, and its members are located primarily in the arms of spiral galaxies. It includes luminous hot stars, main-sequence stars, open star clusters and associated interstellar clouds. Population I objects are relatively metal rich and they are in roughly circular orbits within the plane of the Galaxy. The Sun and its neighbouring stars belong to Population I. Population II has the characteristics of an older generation. Its members are typically evolved stars, with low concentrations of heavier elements. They are found in elliptical galaxies and in the centres and haloes of spiral galaxies. Globular clusters are Population II objects. Sometimes the expression Population III is used for a hypothetical class of objects, belonging to the earliest stage of the life of the Galaxy, which have now completely disappeared. See also: Population I, Population II. |