If someone said, “find the constellation called the Big Dipper,” would you know where to look? You probably would. At least, you think you probably would. Astropotamuses, however, know that no matter how hard you try, you’ll never find it. You see, the Big Dipper, and the Little Dipper, aren’t constellations!
Constellations are recognized by the International Astronomical Union as a collection of stars that belong to the same grouping. Like Leo or Aries or BoÃ¶tes. If you want to talk about a set of stars that make a picture that isn’t part of a constellation, you’re talking about an asterism. If you read my previous post about star hopping, you’ll know that the Big Dipper isn’t a constellation – it’s an asterism. The same with the Little Dipper, too. But that doesn’t mean that they aren’t fun to look at!
The Big Dipper is seven stars that make the shape of a laddle or dipper. In Europe, it’s referred to as the plough. In any case, looks can be deceiving. As we learned by looking at Regulus, some stars as we see them are actually pairs of stars (or more than pairs!) in reality. The stars of the Big Dipper are Dubhe, Merak, Phecda, Megrez, Alioth, Mizar, and Alkaid. Mizar, however, is a just a big surprise when you look closely.
If you have really good eyesight, you might be able to see Alcor with just your eye. It’s Mizar’s binary pair. Binary pairs are connected to each other by gravity and orbit each other. But Mizar is just like Regulus – a pair of binary pairs all in orbit. So there are four stars in Mizar plus a fifth, Alcor. And then, in 2009, an astronomer at the University of Rochester discovered that Alcor is also a binary!
That means that Mizar, as we see it from Earth, is Mizar plus Alcor, plus four other stars! There are the same number of stars in Mizar as there are in the rest of the Big Dipper! Six!!
I’ve included some images of the Big Dipper (minus one star that didn’t fit into the field of view) that were taken the same night as the moon pictures and the images of Regulus. You can see the Big Dipper as the camera sees it with a 30 second exposure (which will see many more stars than your eyes will), plus the same image with six of the seven Big Dipper stars circled (Mizar is double circled), and then a closeup of Mizar and Alcor.
If you look closely at this last one, it shows each of their own binary companions, even though they’re too far away for the camera to resolve properly. Compare their shapes to the shapes of nearby single stars, which are blurred a little because the motor that rotated the camera was not in precise alignment with the motion of the stars. You’ll see Mizar and Alcor are little footballs while the other two are more like lines. This is because the second star of each binary pair lights up more pixels to the side of the main star, so you see a bulge. The motion caused by the camera moving at a different rate than the stars caused the slight “streaking” effect that you see in total.
Oh, and the next time someone points a the Big Dipper and says, “what constellation is that?” You can say, “Ursus Major,” since that’s the constellation that the Big Dipper is a part of. The Little Dipper, by the way, is part of Ursus Minor.