ARCTURUS (Alpha Bootis) - Among the very brightest of stars, shining with a soft orange light, Arcturus lights northern spring skies. It is one of three luminaries that partition the northern sky into very rough 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 Greater 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 class K (K1) giant star with a precisely defined surface temperature of 4290 degrees Kelvin. To the eye, it shines 113 times more brightly than our Sun. Its lower temperature, however, causes it to radiate considerable energy in the infrared. When this infrared radiation is taken into account, Arcturus actually shines almost twice as brightly, releasing 215 times more radiation than our Sun, from which we find a diameter 26 times solar, about a quarter the size of Mercury's orbit.

Arcturus is close and large enough so that its angular diameter of 0.0210 seconds of arc can easily be measured, leading to a very similar direct determination of 25 times the solar dimension and providing nice confirmation of stellar parameters. Arcturus has a velocity relative to the Sun that is higher than other bright stars. Compared with the set of surrounding stars, which orbit the Galaxy on more or less circular orbits, it falls behind by about 100 kilometers per second (as do several others of the "Arcturus Group").

The lagging movement has long suggested that the star comes from an 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. A more intriguing suggestion is that the star actually comes to us from a small galaxy that merged with ours some 5 to 8 billion years ago. 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. Such stars are not expected to have magnetic activity like the Sun, but very weak X-ray emission suggests that Arcturus indeed is magnetically active and has a hard-to-observe "buried corona."

Written by Jim Kaler - Prof. Emeritus of Astronomy, University of Illinois. Read more about STARS...