Celestial News: The Dippers of summer
For Steamboat Pilot & Today
The celestial bears are parading around the North Celestial Pole this month. Better known in the United States as the Big and Little Dippers, these star patterns are recognized and adored by all.
As soon as darkness falls on August evenings, the seven bright stars that form the Big Dipper shine prominently, high up in the northwestern sky. Starting at the tip of the handle, the seven stars of the Big Dipper are named Alkaid, Mizar, Alioth, Megrez, Phecda, Merak and Dubhe.
Dubhe and Merak, the two stars at the end of the Big Dipper’s bowl, are nicknamed “the Pointer Stars” because a line drawn through them points like an arrow to the North Star, Polaris.
Polaris is as bright as the stars of the Big Dipper but is unique because it is the star that lies closest to the north pole of the sky — the North Celestial Pole. As the earth rotates on its axis, the sky appears to spin around the NCP, with Polaris positioned at the center of the bull’s eye. It remains motionless in our sky all night long and can always be relied upon to show us the way north.
Polaris also happens to lie at the end of the handle of the asterism we call the Little Dipper. Like the Big Dipper, the Little Dipper is composed of seven stars. Moving down the handle from Polaris, the names of the remaining stars are Yildun, Epsilon, Zeta, Eta, Pherkad and Kochab. Kochab is nicknamed “the Guardian of the Pole” because it traces a circle around the pole star Polaris, as if protecting it from all harm.
Here are a few things about our Pole Star that maybe you didn’t know:
• Polaris is the closest pulsating Cepheid variable star to earth. Stars like Polaris are well advanced in age and are going through a stage of instability, causing them to beat like a heart. Its brightness waxes and wanes by about 15% over a period of four days but this slight variability is not easily perceptible to the naked eye.
• Polaris is also the brightest member in a trinary star system located 431 lightyears from earth. In other words, what looks like a single star to the unaided eye is really three stars in an intricate dance with each other. One star orbits Polaris with a period of about 30 years and the other takes thousands of years to complete its orbit.
• Polaris has been known by many names down through the ages. Its current popular title comes from the Latin name “Stella Polaris,” meaning “Pole Star.” Other names include Cynosura (the Dog’s Tail), Lodestar, Navigatoria and Angel Stern (Angel Star).
Polaris has not always been our pole star. Thanks to the slow, 26,000-year wobble of the earth on its axis, Polaris assumes the role of our pole star for only a few centuries every cycle before the axis wanders onto another pole star. Polaris will be closest to the pole of the sky in the year 2100. Then, it will start moving away. Better enjoy it while it lasts!
If you have good vision, you can make out an eighth star in the Big Dipper, twinkling right beside Mizar, the star at the crook in the Dipper’s handle. This little star is named Alcor. Mizar and Alcor have been known since antiquity as “the Horse and Rider” and were used as a test for keen eyesight.
In Great Britain, our Big Dipper is imagined to be “Charlemagne’s Wagon.” Mizar is the middle horse pulling the wagon and little Alcor is popularly known as “Jack on the Middle Horse.” So, our rider has a name. It’s Jack!
My favorite story about Alcor comes to us from Viking mythology. To the Vikings, our constellation of Orion the Hunter was Orwandil, the Giant. One day, 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.
Jimmy Westlake is adjunct professor of Physical Sciences at Colorado Mountain College and former director of the Rollins Planetarium at Young Harris College in Georgia and the St. Charles Parish Library Planetarium in Luling, Louisiana. “Celestial News” appears monthly in the Steamboat Pilot & Today.
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