Look Again at the Big Dipper!

If there's one constellation in the Northern Hemisphere that most living there learnt as a child, it's The Plough, or Big Dipper. Truth be told, it's not actually a constellation at all, it's called an asterism -- a pattern of stars seen in Earth's sky which is not an official constellation. Like constellations, they are composed of stars which, while they are in the same general direction, are not physically related,

often being at significantly different distances from Earth. The Big Dipper (named after the huge soup ladle farmers' wives would use to serve up to the farm hands at the end of a busy day's work), is actually part of a sprawling constellation called Ursa Major, home to wonders aplenty in terms of deep sky objects.

Here though, we're going to just concentrate on the Big Dipper itself. To those in North America, Europe and Asia, both it, and indeed Ursa Major itself, is a circumpolar constellation, meaning it is visible every night throughout the year -- its stars never set.

And so to a tour of the stars of this asterism, all visible with the naked eye, in all but the most appalling light pollution. Binoculars will start to reveal its true beauty, and a small telescope will give superb details of the multiple star members.

Starting in the top right hand corner of the bowl of the ladle, Dubhe (Alpha Ursae Majoris (α UMa / α Ursae Majoris)) is the second-brightest star in the constellation of Ursa Major, and is about 124 light years away. It is typical of a red giant, an evolved helium-burning star. It is also a multiple star, orbited by a main sequence companion, Dubhe B, at a distance of about 23 astronomical units (AU), as well as a close pair, Dubhe C, at a distance of about 8000 AU.

Journeying anticlockwise, we come to Merak (Beta Ursae Majoris (β UMa / β Ursae Majoris)). Both it and Dubhe are familiar to northern hemisphere observers as the "pointer stars" in the Big Dipper, and a line connecting them and moving north extends to Polaris, located at the north Celestial Pole in this epoch.

Merak is fairly typical for a main sequence star of its type, although being slightly hotter and larger than our own Sun, it shines several times brighter. The star is surrounded by a cooling disk of dust, much like those discovered around Fomalhaut and most notably Vega. No planets have been discovered orbiting Merak, but the presence of the dust indicates they may exist or be in the process of forming.

Moving to the left of the bowl we come to Phad or Phecda (Gamma Ursae Majoris (γ UMa / γ Ursae Majoris)). It is an average main sequence star not unlike our Sun, although somewhat hotter, brighter and larger.

At the top left of the bowl is Megrez (Delta Ursae Majoris (δ UMa / δ Ursae Majoris)). Megrez has an apparent magnitude of +3.32 making it the dimmest of the seven stars in the Big Dipper. Located 81 light years away, it is a bluish-white main sequence star. It has two faint companions, the 11th magnitude Delta Ursae Majoris B, 190 arcseconds away, and the 10th magnitude Delta Ursae Majoris C, 186 arcseconds away.

Moving to the left, we first come to Alioth (Epsilon Ursae Majoris (ε UMa / ε Ursae Majoris)). It is the brightest star in the entire constellation of Ursa Major, at magnitude +1.76. It is the star in the tail of the bear closest to its body, and thus the star in the handle of the Big Dipper closest to the bowl. Historically, the star was frequently used in celestial navigation in the maritime trade, because it is listed as one of the 57 navigational stars.

Further out along the handle of the Big Dipper, we arrive at Mizar (ζ UMa / ζ Ursae Majoris), 78 light years away. It is a quadruplet system of two binary stars, with an apparent magnitude is +2.23. Its name comes from the Arabic mÄzar, meaning a waistband or girdle.

With normal eyesight one can make out a faint companion just to the east, named Alcor, or 80 Ursae Majoris, at magnitude 3.99. Mizar and Alcor together are sometimes called the "Horse and Rider," and the ability to resolve the two stars with the naked eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars. The two stars lie three light-years apart, and though their proper motions show they move together, it was long believed they do not form a true binary star system, but simply a double star. New data reveals Alcor actually is itself a binary, consisting of Alcor A and Alcor B, and that this binary system is most likely gravitationally bound to Mizar, bringing the full count of stars in this complex system to six.

Lying at the end of the handle is Alkaid (Eta Ursae Majoris (η UMa / η Ursae Majoris)). It has apparent magnitude +1.9, and is a young bluish-white main sequence star, and burning at 20,000 kelvins it is one of the hotter stars visible with the naked eye.

So go and enjoy the Big Dipper, arm yourself with some 10x50 binoculars, and see how many double stars you can resolve in this lovely asterism!

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• TheBigDipper
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• UrsaMajoris
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• DoubleStars
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• AmateurObserving

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