ARCTURUS (Alpha Bootis). Brightest star in the constellation Boötes (the Herdsman) and 4th-brightest star in the entire sky. An orange giant of spectral class K2 III, 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. Arcturus is moving rapidly in the direction of Spica (as seen from the neighborhood of our Sun) with about 90 miles per second. Arcturus approaches the sun at three miles per second. But even this rapid rate escapes our casual notice because of Arcturus' great distance from Earth. Therefore you can comfortably expect to find it every night in the same sky site for centuries to come. As it continues cutting through the star fields of the Milky Way, however, Arcturus will pass our solar system and recede so far from this vicinity that it will fade from naked-eye view in about half a million years.

Arcturus has a distinctly orange tinge compared to its neighbors. The color coordinates with the star's current condition as a "red giant." In other words it has burned through its stores of the simplest element, hydrogen, which is a star's primary fuel. Now Arcturus shines by fusing together atoms of helium. Puffed up by this process, Arcturus has expanded to a diameter some 25 times that of our sun, though it is only a tiny fraction as dense. To the eye, the star shines 113 times more brightly than our Sun. Its lower temperarature, 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. Though it is somewhat brighter than we would expect for a stable helium fusing star, helium fusion to carbon has probably already begun.

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 (Great Bear). Arcturus 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.

Illuminating science, Arcturus in 1635 became the first star ever seen through a telescope in the daytime. A century later Arcturus gave astronomers their first hard lessons about the vastness and strangeness of deep space. When famed British astronomer Edmond Halley reviewed old starmaps in 1718, he noticed that Arcturus was way off course. Halley entertained the idea that Arcturus and other "fixed" stars might actually travel through space. This was a radical idea for his day, when most people believed that only the planets moved against the background of the immutable stars.
[various sources]