Arcturus ärkto͝orˈəs [key], brightest star in the constellation Boötes and 4th-brightest star in the entire sky; Bayer designation Alpha Boötis; 1992 position R.A. 14h15.3m, Dec. +19°13′. An orange giant of spectral class K2 III, it has an apparent magnitude of −0.06; its diameter is about 10 times that of the sun and its luminosity about 100 times that of the sun. Arcturus is one of the nearest giant stars, at a distance of about 36 light-years, and has one of the largest proper motions (annual angular shift in position) of the bright stars. Its name is from the Greek meaning “guardian of the bear,” and it can be found by following the extension of the curve of the handle of the Big Dipper (Large Bear).

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