Mizar and Alcor

A horse and rider gallop across the north and northwest on May evenings. They’re in the handle of the Big Dipper, which is high in the sky at nightfall and low in the northwest at dawn.



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 bygone centuries 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 systems lie about 80 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.



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.



Just how close their relationship is has remained a mystery. Early observations said the systems were perhaps two or three light-years apart. At that range, they probably would not be gravitationally bound to each other.



But observations over the past few years by the Gaia space telescope put the separation at roughly one light-year. That may be close enough for them to be saddled together as a single unit — just like a horse and rider.



Script by Damond Benningfield