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Dipper Twins

March 25, 2016

A horse and rider gallop across the northeastern sky on spring evenings. They form the middle of the handle of the Big Dipper, which is low in the sky at nightfall and circles high overhead in the wee hours of the morning.

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 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.

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. Until recently, the best observations said the two systems were perhaps two or three light-years apart. At that distance, they probably would not be gravitationally bound to each other.

But recent observations put the separation at roughly one light-year. That may be close enough for them to be tied together to form a single unit — just like a horse and rider.

Script by Damond Benningfield

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