In April the Big Dipper is climbing high overhead in the northeast and starting to pour its contents into the Little Dipper – not a very good idea, but fun to contemplate. Meanwhile, the only double star pair where both stars have proper names – Mizar and Alcor – is high in the northeast and ready to challenge your eyesight and boggle your mind.
Mizar is the middle of the three stars that form the handle of the Big Dipper – the same three that we use as an arc to trace a path to Arcturus. (That reference is explained in this month’s “Look East” post.) Wait until an hour or more after sunset, then focus on that center star. Is it one star – or two? For my old eyes, it is one. And since my eyes are not that bad, I question those who say this is an “easy” test of eyesight. But lots of people do indeed see two stars there when they look carefully. Maybe you’re one of them. If you’re not sure, or can see just one, take a look with your binoculars. Now you certainly should see two.
The brighter of the two is Mizar, the fainter one Alcor. More on that in a minute. First, here’s our northern sky for this month.
And here’s what you should see when you look with binoculars at the Big Dipper’s handle.
The words “double star” simply mean that a star that appears as one to our naked eyes, is seen as two when optical aid is used. But they may simply be two stars that are closely aligned, yet in reality very far apart and have no real connection to one another. “Binary star” is the term used for two stars that are gravitationally linked to one another. So here’s the double rub with Mizar:
- When you are looking at Mizar and Alcor, you probably are looking at six stars, not two!
- Scientists still dispute whether Mizar and Alcor are a true double, even though they have been observing this system with telescopes since 1650!
My “sliced bread” reference figures into the Mizar/Alcor picture in a roundabout way. I have trouble remembering things. So when I wanted to remember the approximate distance to Mizar – 80 light years – I asked myself what interesting thing was going on 80 years ago that can help me remember the distance to these stars? And the answer – given a little research – was that about 80 years ago America was introduced to sliced bread all packaged neatly. Actually, sliced bread was first introduced in 1928, according to Wikipedia, but it was in 1930 that the first national marketing campaign began for “Wonder Bread.” Wonderful. So about 80 years ago the light you see left Mizar and Alcor to begin its journey to your eye. Don’t let the different dates bother you because an approximation is close enough.
And Mizar alone is a lot more interesting than sliced bread.
Even a small telescope reveals that Mizar itself is a beautiful double! That’s what was revealed when a telescope was turned on it in 1650. But no telescope can reveal to the eye that these two stars are in fact, each a double! The stars in each pair are so close to one another that only an instrument known as an interferometer can reveal them. So what we see as Mizar is in fact four stars. (Double stars are a special love of mine, and I wrote about observing Mizar in the double star blog I share with John Nanson here.)
But what about Alcor? The Hipparchos satellite, the best modern source for star distances, found Mizar to be 78.1 light years away and Alcor to be 81.1. Those are great ball park figures and good enough for the sliced bread reference. But they may be wrong. The astronomer James Kaler wrote a few years ago in his book “The Hundred Greatest Stars” that these distances may be wrong – in fact, some evidence suggested then that Mizar was actually farther away than Alcor. Kaler concluded in his book that they are “probably paired.”
But now comes more evidence as reported in the current (2010) Wikipedia reference to Mizar:
. . . in 2009, it was reported by astronomer Eric Mamajek and collaborators that 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.
So what our naked eye reveals as one or two stars, may indeed be a complex system of six stars! Which in my mind says that slicing up Mizar and Alcor this way may be – well, may be the best thing since sliced bread and just the sort of thing that makes observing the stars such a treat for the eye and mind!