Alcor is a double star in the costellation Ursa Major, formed by a white star and a red dwarf. It's very close to another bright star (Mizar) making it difficult to detect. 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. Arabic literature says that only those with the sharpest eyesight can see the companion of Mizar. It lies about 83 light-years away from Earth.