Mizar and Alcor
You would think that one of the best known stars in the sky would be completely understood. Think again. The most famous of double stars is Ursa Major’s naked eye pair Mizar and Alcor the only double for which each member has a name. Mizar has a magnitude of 2.0 while Alcor has a magnitude of 4.0.
It is said that the Arab astronomers of 1,000 years ago used the Mizar/Alcor system as a way of testing people’s eyesight. Haze in the sky can it make it difficult to see Alcor.
However there is a slight mystery because some astronomers wonder if they are they a true double system, Mizar lies at a distance of 78 light years while Alcor lies at a distance of 81 light years. If these results are suspect to doubt and they are a true pair then their separation from each other is about 0.27 light years meaning that they take around 0.75 million years to orbit each other.