The Big Dipper is still riding high in the Washington evening spring skies and unless you’re in the middle of a shopping mall parking lot, you should easily see it hanging upside down in the high northern sky. Even though it’s probably the most recognized star picture in the sky, the Big Dipper is not officially one of the 88 constellations that can be seen from Earth. The seven bright stars that make up the Big Dipper are, however, part of Ursa Major, otherwise known as the Big Bear. The Dipper is by far the brightest part of the constellation, outlining the rear end and tail of the bear.
The Big Dipper all by itself possesses many celestial treasures. It even has a built-in eyesight test that’s been used by many societies and cultures dating back to ancient times. See if you can pass the test. Just look at Mizar, the bright star that marks the middle star in the handle. If you’ve been eating your carrots and not staring too much at the TV you should easily be able to see that Mizar has company. Right next to it is Alcor, a star only about one-fifth as bright as Mizar. If you can’t see Alcor without a lot of squinting, schedule a visit to an eye doctor.
In Arabic culture, these two stars are known as the “horse and rider,” with Mizar being the horse and Alcor being the rider, although something’s a little screwed up here. Like many of the brighter stars, Mizar and Alcor have Arabic names and Alcor, the rider, actually translates to English as “black horse,” but Mizar means “groin of the bear,” which refers to its parent constellation.
Alcor and Mizar are what astronomers consider an optical double-star system, because while the two stars are close to each other in the sky they have no physical attraction to each other. They just happen to be in the same line of sight. Mizar is 78 light years from Earth and Alcor is 81 light years away, with one light year equivalent to nearly six trillion miles. Many double stars in the heavens are considered binary stars because they’re actually in orbit around each other. What appears to be a single star to you with your naked eye may actually be four, five or even more stars, all in complicated orbital schemes around each other.
In fact, if you aim a small to moderate telescope at Alcor and Mizar you’ll actually see four stars. This is a good test for your telescope. If your scope is in good shape you’ll see that Mizar is actually a close double star all by itself, and right next to Alcor and the Mizar double is a fourth star called Sidus Ludoviciana.
Through much more sophisticated telescopes and other advanced optics, there are even more stars that make up the horse and rider. Mizar is actually a quintuple star system and Alcor has a small companion star. Our “horse and rider” is really a team of five horses and two riders riding the trails of heaven.
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and author of the new book “Washington Starwatch,” available at bookstores and on his Web site, www.lynchandthestars.com.
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