All spring, as darkness has fallen, the bright star Arcturus has seemed to climb gradually higher and higher in the eastern sky. Now we see it nearly two thirds of the way to the zenith a half hour after sunset. Arcturus is the brightest star in the constellation of Boötes the Herdsman so it is also called Alpha Boötis. The pattern of stars in this constellation forms the shape of a kite or an ice cream cone; seeing a Herdsman driving the bears around the sky requires a great deal more imagination than I've ever been able to muster.
Arcturus is believed to be one of the first stars named by ancient observers. It is translated as "Guardian of the Bear" and is a name that was once used for the entire constellation of Boötes. It is easily found by noting that the curve of the handle of the Big Dipper is part of a circle -- an arc -- and we can just "follow the arc to Arcturus."
Arcturus, at -0.05 magnitude, is the fourth brightest star we see in the night sky. Only Sirius, Canopus, and Alpha Centauri are brighter. Canopus and Alpha Centauri are not visible from our latitude, so the second brightest star visible from Morgantown is this Guardian of the Bear.
Arcturus is a giant with a diameter about 18 times our Sun's and four times as much mass. Its surface temperature is about 1500 degrees lower than Sun's but its much greater surface area results in an outpouring of energy at a rate making it 105 times as luminous as Sun. Recent observations by the European Space Agency's Hipparcos Space Astrometry Mission have revised Arcturus's distance to 36.7 light years from us.
Arcturus has the largest "proper motion" -- motion across the sky -- of any of the bright stars except Alpha Centauri, the nearest star to our solar system. In 100 years Arcturus moves across the sky a distance equal to about half the width of your little finger held at arm's length. At its distance of nearly 37 light years, this motion, when combined with its motion along our line of sight measured spectroscopically using the Doppler shift, yields a space velocity of about 76 miles per second with respect to our Sun. Most stars in our vicinity are moving relatively slowly with respect to Sol because of our common motion carrying us around the center of the Milky Way galaxy every 250 million years. Arcturus is in an elongated orbit around the Galaxy's center that carries it out into the Galaxy's halo.
It was formed in the halo of the Milky Way and is an interloper in our neighborhood. It has been visible to the naked eye for only about half a million years. It will be a little closer in a few thousand years, but then will recede from our view in another half million years as it continues its journey on a different orbital path.
Arcturus's great brilliance makes it possible to obtain very detailed spectra and to determine its chemical composition. Arcturus is deficient in elements such as silicon, aluminum, and iron which are formed in stars. It contains only about a fifth as much of these elements as Sun. These elements are formed inside stars, mixed into the interstellar medium as stars explode, and incorporated into subsequent generations of stars. This chemical composition reinforces our identification of Arcturus as a member of the galactic halo. It was formed about 10 billion years ago, in a generation of star formation prior to that in which our Sun was formed, before the interstellar medium could be enriched in elements such as silicon and iron. Arcturus, about twice as old as Earth and the solar system, is the oldest thing most of us have ever seen, and is the oldest object easily visible to the naked eye.
