Amazing stars: Mizar and Alcor
Giles Sparrow reveals the secrets of this intriguing stellar system in the Plough asterism
Stars, like bobbies on the beat, have a tendency to come in pairs. Born in large clusters from collapsing clouds of interstellar gas, they’re gregarious by nature, and many emerge from the turbulent breakup of their birth cluster still locked in orbit with one or more companions. Most of these stellar pairings are too closely bound (or too far away) to be seen with even a powerful telescope, but one particular pair is easy to spot with the naked eye alone. What’s more, this famous stellar pairing played a key role in many of our discoveries about the behaviour of binary and multiple stars.
Look north on winter evenings and you’ll see the familiar pattern of the Plough or Big Dipper rising up over the horizon. Four of the brightest stars in the constellation of Ursa Major, the Great Bear, form a distinctive ‘bowl’ or ‘blade’, while three more make a curving handle. Even a casual glance at Mizar (Zeta
(c Ursae Majoris), the middle star of this handle, should reveal something unusual about it – a fainter companion, Alcor (80 Ursae Majoris), separated by about one-third the width of a full Moon.
Such a close pairing might seem to immediately mark the two stars out as a genuine binary star (a pair bound together by gravity in orbit around each other), but the link remained unproven for a long time. It wasn’t until 1869 that Richard Anthony Proctor discovered that both stars are moving in the same