At around 10 p.m. local daylight time on these warm June evenings, face north and look overhead to see the seven stars that compose the famous Big Dipper. At this time of the year, the handle appears to stand almost straight up while the bowl appears tilted down; draining its unknown contents toward the right. The Dipper is not a constellation in of itself, but an asterism, a prominent pattern or group of stars, typically having a popular name, belonging to a specific constellation.
For most sky gazers, the Big Dipper is probably the most important group of stars in the sky. For anyone in the latitude of New York (41 degrees North) or points northward, it never goes below the horizon. It is one of the most recognizable patterns in the sky and thus one of the easiest for the novice to find. Of greatest importance is the ability to utilize the Big Dipper to locate Polaris, the North Star.
This is made possible by the two bright stars that mark the outer edge of the bowl of the Big Dipper. These two stars — Dubhe and Merak — are known as the Pointers, because they always point to Polaris. Just draw a line, in your imagination, between these two stars and prolong this line about 5 times, the way our map shows; this line will ultimately hit a moderately bright star. That will be Polaris.
Even those who live well south of the equator can see the Big Dipper now. During autumn in the Southern Hemisphere, it appears to hover upside-down above the northern horizon for those down to about latitude 30-degrees south. That includes the northern two-thirds of South America and virtually all of Africa. In his classic constellation guide, “The Stars — A New Way to See Them,” author H.A. Rey sketched a cute cartoon on page 24 of a mother kangaroo and its joey above the caption: “The Dipper? Never seen it …”
And yet for the northern two-thirds of Australia, during April, May and June, the Dipper is plainly visible hanging low above the northern horizon.
Celestial eye test
The middle star in the Dipper’s handle is Zeta Ursae Majoris; the Greek letter is the given name, and the possessive of the Latin name of its constellation. In this case Ursa Major, the Great Bear — is its family name.
But this particular star is far better known as “Mizar,” derived from the Arabic meaning for “wrapping” or “cover.”
With just your eyes alone, if you look carefully, you’ll notice that Mizar appears to have a fainter companion by the name of Alcor, also derived from the Arabic, meaning “faint one.” The Persian Al Kazwini stated during the 13th century that people “tested their eyesight by this star.”
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Mizar and Alcor were known as the “Horse and Rider” to the ancient Arabs and the Arabian writer Al Firuzabadi, in the 14th century referred to Alcor as Al Sadak, “The Test” or “The Riddle.” So, when the ancient Arabs remarked: “He can see Alcor,” they were alluding to someone who had normal vision. Why not test your own eyesight by looking for Alcor tonight?
Alcor is one-fifth as bright as Mizar and the two stars are separated by roughly one-third of the apparent diameter of the moon. Both stars are located about 83 light-years away and recent observations suggest that Alcor is traveling around Mizar in a wide orbit that takes 750,000 years to complete.
Binoculars provide a better view of this pair, but there’s more to see here than many skywatchers realize.
Two stars for the price of one
As an example, Mizar, is in itself a true binary star; one of the first to be observed and identified as a binary pair by astronomers in the 17th century. A binary star is two or more stars that are physically connected with each other and are “locked” or connected together by gravity.
They actually orbit each other in a fashion similar to the Earth and the planets orbiting the sun or the moon orbiting the Earth. To see the companion of Mizar you’ll need a small telescope and an eyepiece providing a magnification of at least 50-power.
An ersatz planet
Between Mizar and Alcor and a little to their south is an 8th-magnitude star that was once mistaken for a new planet. In December 1722, the German professor Johann Georg Liebknecht announced that this object had just moved into its present position. He christened his “planet” Sidus Ludoviciana — Ludwig’s Star — to flatter his local sovereign Ludwig V of Hesse-Darmstadt in hopes of being granted money.
The object never moved again.
Liebknecht disappeared in a hail of ridicule, but amazingly the name of the “faux” or “ersatz” planet remains to this very day!
Giant star and a big galaxy
If you draw a line east from Mizar through Alcor, you’ll come across a 5th-magnitude star, glowing with a distinct ruddy hue. That’s 83 Ursae Majoris, a red giant star roughly 80 times larger and about 1,300 times more luminous than the sun. It’s located at a distance of 580 light-years from us.
If you continue onward along the same line at about twice the distance past 83, you’ll come to the big, but dim galaxy M101. Under a dark sky M101 appears in 7 x 50 and larger binoculars in much the same way that its discoverer Pierre Méchain described it in 1781: “very obscure and pretty large.” It is popularly known as the “Pinwheel Galaxy,” and at 21 million light years, it is one of the closer spiral galaxies to our Milky Way. But in order to perceive its spiral structure, you’ll need a rather large telescope, very dark skies and a low-power eyepiece.
Farewell to the Dipper
Finally, our familiar Dipper will not last forever. Of the seven stars that make up this pattern, five apparently belong to a loosely joined swarm of stars, all hurtling through space at roughly the same speed and the same direction. These five range in distances from 78 to 84 light-years from Earth. Two of the stars, however — Dubhe (the northern Pointer) and Alkaid (the star at the end of the handle) — are not part of the swarm and appear to be rushing at even greater speeds in the opposite direction. Alkaid is 101 light-years away, while Dubhe is 124 light-years distant.
These opposing motions will slowly alter the form of the Big Dipper. In short, the Dipper is slowly going to pieces. The bent handle will bend still more as time wears on, while the bowl will spread. Granted, it will still remain a fairly convincing dipper for about the next 25,000 years, but 50,000 years (or 2,000 generations) from now it will be hopelessly out of shape.
If you’re looking for a telescope or binoculars to observe the night sky, our guides for the best binoculars deals and the best telescope deals now can help. Our guides on the best cameras for astrophotography and best lenses for astrophotography can also help you prepare to capture the next skywatching sight.
Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope and other publications.
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