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ARCTURUS
Among the very brightest of stars, shining at magnitude zero with a soft orange light, Arcturus lights northern spring skies. It is one of three luminaries that partition the northern sky into thirds, the others being summer's Vega and winter's Capella. Of the three, Arcturus, the Alpha star of the constellation Bootes, the Herdsman, is slightly the brighter, making it the brightest star of the northern hemisphere and the fourth brightest star of the entire sky, following only Sirius, Canopus, and Alpha Centauri. Arcturus, the "Bear Watcher," follows Ursa Major, the Great Bear, around the pole, "arktos" being the Greek name for "bear," from which our word "arctic" is derived by reference with the constellation of the Great Bear. Arcturus is located at a distance of 37 light years, and became famous when its light was used to open the 1933 world's fair in Chicago, as that light had left the star at about the time of the previous Chicago fair in 1893. It is a classic orange giant star with a precisely defined surface temperature of 4290 degrees Kelvin. Taking infrared radiation into account, it shines 180 times more brightly than our Sun, from which we find a diameter 24.5 times solar, about a quarter the size of Mercury's orbit. Arcturus is close and large enough so that its angular diameter of 0.0191 seconds of arc can easily be measured, leading to a direct determination of 23 times the solar dimension and providing nice confirmation of stellar parameters. Arcturus has a velocity somewhat higher than other bright stars, and comes from a somewhat older population of the Galaxy. Consistently, it is somewhat deficient in metals, having only about 20 percent as much iron relative to hydrogen as found in the Sun. As a giant, weighing in at around 1.5 times the mass of the Sun, it has ceased the fusion of hydrogen in its core. Though it is somewhat brighter than we would expect for a stable helium fusing star, helium fusion to carbon has probably already begun.