An Old Friend

Daily circling the Pole, the Big Dipper is visible in Vermont all night, every night, and as spring advances, it is rising ever higher. It is an asterism, part of a large constellation that is easy tyo recognize. That constellation is Ursa Major, the Great Bear.

There is a wonderful story in Greek mythology about how the bear got into the sky. Zeus fell in love with the mortal woman Callisto; they had a son together, named Arcas. His wife Hera, furious at this infidelity, transformed Callisto into a lumbering bear.

When he grew to manhood, Arcas became a great hunter. Not recognizing Callisto (his own mother!) in her changed form, he was about to slay her. Happily, though, Zeus intervened, changed Arcas into a little bear, and, swinging the two bears by their tails, flung them into the sky. This explains not only their presence in the sky but also their abnormally long tails.

Long before people studied the stars through telescopes, they thought that the central star of the tail, the star Mizar, was really a "double star." Its companion, Alcor, can be seen by a keen-eyed observer; seeing them together was a test of eyesight used by many peoples. Together, they are called "the horse and its rider."

But, though the pair looks like what we all think of as a double star, it is actually just a ringer. There is really no real connection between the two stars. The pair is called an "optical double;" it is only our relative position that makes them appear related.

Like "the horse and its rider," most constellations, are just chance alignments of stars; as they move through space the observed pattern will change. Not so for the Big Dipper. Except for the two stars at the extreme ends of the Dipper, the other stars move together through space. The pattern will always remain the same. They form the nearest star cluster, the Ursa Major Moving Cluster, moving at about nine miles per second towards the constellation of Sagittarius. Pretty fast for an old bear.

(04/04/07)