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red giant, star that is relatively cool but very luminous because of its great size. All normal stars are expected to pass eventually through a red-giant phase as a consequence of stellar evolution. As a star uses up its hydrogen by converting it to helium, its central core contracts while the outer layers expand and cool; this process produces the low temperature and large size (from 10 to 100 times that of the sun) that characterize the red giant. Although most giant stars are red, some prominent giant stars are other colors near the red end of the spectrum, e.g., Arcturus (orange), Aldebaran (orange), and Capella (yellow). The largest and brightest stars (excluding supernovas) are classed as supergiants. These stars have diameters between 100 and 1,000 times that of the sun. Blue supergiants, e.g., Rigel, are young stars on the main sequence of the Hertzsprung-Russell diagram, whereas red supergiants, e.g., Betelgeuse and Antares, are old, highly evolved stars.
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