Kaler, James B. Department of Astronomy, College of Liberal Arts and Sciences, University of Illinois, Urbana-Champaign, Illinois.
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A member of a class of evolved stars that occupy the top of the Hertzsprung-Russell diagram to the right of the main sequence. The absolute visual magnitudes (MV) of supergiants range approximately between −4 and −10, and they are the largest and brightest stars. They are recognized by their spectroscopic characteristics. For example, those in class A have narrow hydrogen lines. They were first identified as distinct by Ejnar Hertzsprung when he found that Williamina Fleming's narrow-line “c” class had low proper motions, showing the stars to be far away and luminous. The “c” designation is still used to denote supergiants, though the use of luminosity class I is much more common. Supergiants are subdivided into classes Ib (MV about −5) and Ia (MV about −7). A “hypergiant class zero” was later added near MV = −10; the use of transition class Ia–0 at absolute visual magnitudes near −9 is now common. Red supergiants are the largest of all stars, and at maximum can reach diameters approaching that of the orbit of Saturn. The apparently brightest supergiants are Betelgeuse and Antares; among the largest known, VV Cephei and Mu Cephei.
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