Mizar and Alcor 
A horse and rider gallop high across the northern sky on summer evenings. They form the middle of the handle of the Big Dipper.
They're the stars Mizar and Alcor. Mizar is the brighter of the two, with fainter Alcor just a whisker away. They're so close together that the skywatchers of ancient Arabia thought of them as a horse and rider.
Mizar is a system of four stars, all of which are a little hotter and brighter than the Sun. Alcor consists of two stars -- one of them more impressive than the Sun, the other less impressive. Both of these systems probably lie between 80 and 85 light-years from Earth.
One question that astronomers have asked for centuries is whether the two systems are bound to each other, or if they just happen to line up in the same direction in the sky.
We know for sure that they're both members of a wide-spread cluster. That makes Mizar and Alcor stellar siblings -- they formed at the same time, from the same cloud of gas and dust.
But just how close their relationship is remains a bit of a mystery. The best observations say they're separated by two or three light-years, which would mean that they aren't gravitationally bound to each other. But there's a lot of wiggle room in Mizar's distance. That means it's possible that the Mizar and Alcor systems are separated by just a quarter of a light-year. At that distance, they would be tied together to form a single unit -- just like a horse and rider.
Script by Damond Benningfield, Copyright 2010