red giant

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 1,500 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. 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.

red giant A cool, large, and highly luminous star. Red giants are stars that have left the main sequence, having exhausted the hydrogen fuel in their cores, and are powered by nuclear reactions between elements heavier than hydrogen. They owe their colour to their cool surface temperatures (below 4000 K, spectral type K or M). Because of their large diameters, 25 times that of the Sun or more, they are highly luminous, many hundreds of times that of the sun. Massive main-sequence stars may evolve into red giants after passing through a blue-giant phase, while lower-mass stars, such as the Sun, will evolve directly into a red giant. Many red giants are pulsating variable stars.

red gi·ant • n. Astron. a very large star of high luminosity and low surface temperature. Red giants are thought to be in a late stage of evolution when no hydrogen remains in the core to fuel nuclear fusion.