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Altair (α Aquilae, α Aql) is the brightest star in the constellation Aquila and the twelfth brightest star in the night sky. It is currently in the G-cloud.〔http://interstellar.jpl.nasa.gov/interstellar/probe/introduction/neighborhood.html, Our Local Galactic Neighborhood, NASA〕〔http://www.centauri-dreams.org/?p=14203, Into the Interstellar Void, Centauri Dreams〕 Altair is an A-type main sequence star with an apparent visual magnitude of 0.77 and is one of the vertices of the Summer Triangle (the other two vertices are marked by Deneb and Vega).〔〔〔(Summer Triangle ), entry, ''The Internet Encyclopedia of Science'', David Darling. Accessed on line November 26, 2008.〕 It is 16.7 light-years (5.13 parsecs) from Earth and is one of the closest stars visible to the naked eye. Altair rotates rapidly, with a velocity at the equator of approximately 286 km/s.〔From values of ''v'' sin ''i'' and ''i'' in the second column of Table 1, Monnier et al. 2007.〕〔 A study with the Palomar Testbed Interferometer revealed that Altair is not spherical, but is flattened at the poles due to its high rate of rotation.〔Altair's Oblateness and Rotation Velocity from Long-Baseline Interferometry, Gerard T. van Belle, David R. Ciardi, Robert R. Thompson, Rachel L. Akeson, and Elizabeth A. Lada, ''Astrophysical Journal'' 559 (October 1, 2001), pp. 1155–1164, , .〕 Other interferometric studies with multiple telescopes, operating in the infrared, have imaged and confirmed this phenomenon.〔Imaging the Surface of Altair, J. Monnier ''et al.'', ''Science'' 317, #5836 (July 20, 2007), pp. 342–345, , , . Accessed on line November 25, 2008. See second column of Table 1 for stellar parameters.〕 ==Characteristics== Altair is located 16.7 light-years (5.13 parsecs) from Earth and is one of the closest stars visible to the naked eye.〔p. 194, ''The Brightest Stars: Discovering The Universe Through The Sky's Most Brilliant Stars'', Fred Schaaf, Hoboken, New Jersey: John Wiley & Sons, Inc., 2008, ISBN 978-0-471-70410-2.〕 Along with Beta Aquilae and Gamma Aquilae, it forms the well-known line of stars sometimes referred to as the ''Family of Aquila'' or ''Shaft of Aquila''.〔p. 190, Schaaf 2008.〕 Altair is a type-A main sequence star with approximately 1.8 times the mass of the Sun and 11 times its luminosity.〔〔 Altair possesses an extremely rapid rate of rotation; it has a rotational period of approximately 9 hours.〔 For comparison, the equator of the Sun requires a little more than 25 days for a complete rotation. This rapid rotation forces Altair to be oblate; its equatorial diameter is over 20 percent greater than its polar diameter.〔 Satellite measurements made in 1999 with the Wide Field Infrared Explorer showed that the brightness of Altair fluctuates slightly, varying by just a few thousandths of a magnitude with several different periods less than 2 hours. As a result, it was identified in 2005 as a Delta Scuti variable star. Its light curve can be approximated by adding together a number of sine waves, with periods that range between 0.8 and 1.5 hours.〔Altair: The Brightest δ Scuti Star, D. L. Buzasi et al., ''The Astrophysical Journal'' 619, #2 (February 2005), pp. 1072–1076, , .〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Altair」の詳細全文を読む スポンサード リンク
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