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July Star of the Month: Mizar

Of the seven stars that make up the Big Dipper, the most famous might be Mizar, a star glorified in the annals of astronomical history many times over. To find it, look first for the Big Dipper in your northwestern sky. At nightfall, note that the three stars comprising the handle stand upright over the bowl. Mizar, the middle handle star, shines one star down from Alkaid, the star illuminating the end of the handle.

On a clear and dark night, you can see Mizar's companion star, Alcor. This naked-eye double star once served to test people's eyesight in olden times. If you can't see Alcor with the eyes alone, you should be able to see the star rather readily through binoculars. Often referred to as the "rider of the horse," Alcor is sometimes regarded in folklore as the missing Pleiad from the Pleiades star cluster.

Apart from Alcor, Mizar itself is a double, which in 1650 became the first double star to be seen through a telescope. Although astronomers now know that Mizar is a true binary star -- two stars orbiting one another -- few, if any, astronomers back then even dreamed that doubles were anything other than chance alignments of physically unrelated stars.

Some binary stars, it was later discovered, hover so close together that they can't even be seen through a telescope. In 1889, an instrument known as a spectroscope revealed that Mizar's brighter telescopic component consisted of two stars -- introducing astronomy to the first binary star ever discovered by spectroscopic means. At a later date, Mizar's dimmer telescopic component also showed itself to be a spectroscopic binary; and lo and behold, astronomers came to realize that Mizar was a double-double, or a quadruple star!

Professor Kaler of the University of Illinois, a star aficionado, reports that Mizar and Alcor are probably gravitationally bound as well, which would up the count of this multiple star system to five, making it a quintuple star!

copyright 2003 by Bruce McClure

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