Jimmy Westlake: Spot the ‘Horse and Rider’
May 26, 2014
Steamboat Springs — The Big Dipper dominates our springtime sky just as Orion dominates our winter sky. It's hard to miss this time of year — just go outside after darkness falls and look straight up. There, you'll find the seven bright stars of our beloved Big Dipper.
The two stars at the end of the Dipper's bowl are Dubhe and Merak, the so-called "pointer stars." A line drawn through them and extended out of the top of the Dipper always will lead you 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 Dipper's bent handle: Alioth, Mizar and Alkaid.
If you have good vision, you can make out an eighth star in the Big Dipper, right beside Mizar, the star at the crook in the Dipper's handle. This little star is Alcor. Mizar and Alcor have been known since antiquity as the “Horse and Rider."
In Great Britain, our Big Dipper is known as "Charlemagne's Wagon." Mizar is the middle horse pulling the wagon, and Alcor is popularly known as "Jack on the Middle Horse." So, our rider has a name: it's 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 each other is still a matter of debate among astronomers. If they do, then their orbital period must be hundreds of thousands of years.
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In the year 1650, when astronomers first aimed their telescopes at the famed "Horse and Rider," they were astonished to discover that the Horse, Mizar, itself appeared double. Almost any small telescope will reveal the twin stars of Mizar, called Mizar A and B.
Modern astronomers have discovered that each of Mizar's twin stars is again double. This amazing quintuple star system is located about 80 light years from Earth.
My favorite story about Alcor comes from Viking mythology, in which our constellation of Orion the Hunter was called Orwandil the 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 the toe into the northern sky where we can see it shining tonight, right beside Mizar, as our little star Alcor.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in the Steamboat Today newspaper and his “Cosmic Moment” radio spots can be heard on local radio station KFMU. Check out Westlake’s astrophotography website at http://www.jwestlake.com.