Stars on the horizontal branch are hotter, with only a small range of luminosities around 75 L ☉. Red-giant-branch stars have luminosities up to nearly three thousand times that of the Sun ( L ☉), spectral types of K or M, have surface temperatures of 3,000–4,000 K, and radii up to about 200 times the Sun ( R ☉). Despite the lower energy density of their envelope, red giants are many times more luminous than the Sun because of their great size. However, their outer envelope is lower in temperature, giving them a yellowish-orange hue. They have radii tens to hundreds of times larger than that of the Sun. The K0 RGB star Arcturus is 36 light-years away, and Gamma Crucis is the nearest M-class giant at 88 light-years' distance.Ī red giant will usually produce a planetary nebula and become a white dwarf at the end of its life.Īn illustration of the structure of the Sun and its possible future as a red giant, comparing their structure and size.Ī red giant is a star that has exhausted the supply of hydrogen in its core and has begun thermonuclear fusion of hydrogen in a shell surrounding the core. Many of the well-known bright stars are red giants because they are luminous and moderately common.
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