Jimmy Westlake: The Horse and the Rider
March 16, 2010
Steamboat Springs — Spring begins this year in the northern hemisphere at 11:32 a.m. Saturday. Signs that spring has arrived in the Rockies include the swarms of red-winged black birds that will soon fill the air and the return of the Big Dipper to our early evening sky.
Look to the northeastern sky to find the seven bright stars of the Big Dipper, propped up on its handle. The two stars marking the top end of the Dipper's bowl are named Dubhe and Merak. These are the so-called "pointer stars," because a line drawn through them and extended out of the top of the Dipper will lead you directly to our North Star, Polaris. Completing the Dipper's bowl are two more stars named Phecda and Megrez. Megrez is the star positioned right where the handle of the Big Dipper joins the bowl. The remaining three stars form the bent handle of the Big Dipper: Alioth, Mizar and Alkaid at the end of the handle.
If you have good vision, you can make out an eighth star in the Big Dipper, right beside Mizar, the star at the bend in the Dipper's handle. Although it is the faintest of the stars outlining the Big Dipper, it is no less famous than the brighter seven. Named Alcor, it has been known since antiquity as "the Rider," and its neighbor Mizar as "the Horse." In Great Britain, where our Big Dipper is known as "Charles's Wain," Mizar is considered the middle horse pulling the wain (wagon), and Alcor is popularly known as "Jack on the Middle Horse." So, our rider has a name: Jack.
Mizar and Alcor together form one of the easiest and best-known naked-eye double stars in the heavens. Whether or not the two stars actually orbit around each other is still a matter of debate among astronomers. If they do, then their orbital period must be measured in hundreds of thousands of years.
In the year 1650, when astronomers first aimed their telescopes at the famed "Horse and Rider," they were astonished to discover that the Horse itself appeared double. Almost any small telescope will reveal the twin stars of Mizar, called Mizar A and B. Modern astronomers have used sophisticated techniques to discover that each of Mizar's twin components is again double! This amazing quintuple star system is located about 80 light years from Earth.
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My favorite story concerning the star Alcor comes from Viking mythology, where we find that our constellation Orion the Hunter was called Orwandil and the bright star Rigel represented the foot of this celestial giant. Once, when Orwandil was crying like a big baby because his toe was frostbitten, the god Thor grew tired of his whining and snapped off the frozen toe. Yee-ouch! He then threw it into the northern sky where we can see it today shining right beside Mizar as our little star Alcor.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all around the world. Check out Jimmy's Web site at http://www.jwestlake.com.