McAlister, Harold A. Center for High Angular Resolution Astronomy, Georgia State University, Atlanta, Georgia.
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A cool and highly luminous star, prominently located in the right shoulder of the constellation Orion and noticeably red in color. Betelgeuse, or α Orionis, is a supergiant star about 130 parsecs (430 light-years) from the Sun. Its spectral type of M2 indicates an effective temperature of approximately 3500 K (5800°F). This temperature would result in a low overall luminosity were it not for the enormous diameter of the star, about 1100 times that of the Sun. Replacing the Sun, Betelgeuse would fill the solar system to beyond the orbit of Mars. Betelgeuse is a supergiant star with approximately 150,000 times the Sun's luminosity. Betelgeuse varies in brightness with a period of 6.4 years. Its mean density is extremely low, less than one-millionth the density of water, and the low surface gravity of the rarefied outer layers results in a continual loss of matter, presently at the rate of 1 solar mass every 250,000 years. Observations at infrared and radio wavelengths show a complex system of dust and gas shells extending out as far as a thousand stellar radii. If this material were visible to the eye, Betelgeuse would appear as large as Mars through a telescope. These shells appear to originate from occasional outbursts in which material is ejected and eventually merges with the interstellar medium. See also: Spectral type; Stellar evolution; Supergiant star; Variable star
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