Mizar and Alcor [Photo: Fred Espenak]

The Big Dipper – called the Plough in Britain – is part of the constellation Ursa Major. Its seven stars are probably the best known star group in the sky. [image: Yuri Beletsky (Carnegie Las Campanas Observatory, TWAN)]

But are there really only seven stars in the Big Dipper?

The Arab eye test
The middle star of the handle of the dipper is the bright star Mizar, also known as Zeta Ursae Majoris. If we look carefully at Mizar, we can see that it's a slightly odd shape. That's because a second, and fainter, star is in the same line of sight. Its name is Alcor, and sometimes they're called the Horse and Rider. The Big Dipper has eight stars.

Al-Sufi (903-986), the great Persian astronomer, described how Mizar and Alcor were used as an eye test. George M. Bohigian, a doctor specializing in ophthalmology, used a small group of individuals to compare the results of the “Arab eye test” with a modern one. He found that they gave similar results. The dipper has eight stars.

And it wasn't only in the skylore of the Persians and the Arabs that Mizar and Alcor appeared. In both Germany and England they were dubbed the Horse and Rider. They're also part of Indian astronomy and Japanese mythology.

The first telescopic binary
In 1617 Italian astronomer Benedetto Castelli discovered the first telescopic binary star. It was Mizar. Although he was excited enough to ask Galileo to have a look, neither of them realized that it was a true binary. And for centuries, astronomers generally assumed that double stars were nothing more than stars that happened to appear close together in our line of sight.

Mizar A and Mizar B are widely separated, sometimes as much as 380 AU apart, taking some 3000 years to orbit each other. (The AU is an astronomical unit, equal to the Earth-Sun distance.) Although you can't resolve Mizar A and Mizar B with the unaided eye, even the primitive telescopes of the early seventeenth century could do it. So the Big Dipper had nine stars, but it takes a telescope to see all nine.

The first spectroscopic binary
About 270 years after Castelli asked Galileo to check out Mizar, at Harvard College Observatory they were studying Mizar with a spectroscope. The observatory was compiling a spectroscopic star catalogue on behalf of Anna Draper in memory of her husband Henry Draper. Some confusion about the doubling of one of the spectral lines alerted the astronomers to the existence of a second star. We call a star that can't be seen, but can be detected from its spectrum, a spectroscopic binary.

The new discovery meant that Mizar was a triple star. But there was more, for in 1908 two independent observers discovered that Mizar B was also a spectroscopic binary, making Mizar a quadruple star system. That also upped the overall star count for the Big Dipper to eleven.

Alcor
Alcor is something of a poor relation. American astronomer Jim Kaler described it as “forever tied to Mizar”. Yet although it's the junior partner in “Mizar and Alcor”, Alcor did finally have its moment of glory. In 2009 two separate observing teams, using different methods, announced that Alcor is a spectroscopic binary.

Alcor A, the star we can see, is about twelve times brighter than the Sun. Its companion Alcor B, which orbits very close to it, is a red dwarf.

So the Horse and Rider contains at least six stars, not two, and gives the Big Dipper an even dozen stars. But what about the Mizar quadruple and the Alcor binary systems? Are we sure that their closeness is illusory? Could they actually form a sextuple star system?

The Ursa Major Moving Cluster
In 1869 English astronomer R.A. Proctor recognized that many of the Ursa Major stars had the same proper motion. This is their motion against the background of fixed stars, and represents their actual motion, not any apparent motion as the Earth moves. Since then, astronomers have also observed that the stars have the same radial velocity and are at about the same distance from us. Radial velocity is a measure of movement in our line of sight. Distance is calculated using parallax.

From this evidence, astronomers conclude that most of the Big Dipper stars, and a number of others in Ursa Major, probably originated in the same star cluster some 300 million years ago. Much of the cluster has dispersed, but not these remnants. Because they're moving together, it makes it harder to say whether Mizar and Alcor are, in addition, bound gravitationally to form a sextuple star system.

Some people now state that there is a Mizar-Alcor sextuple system. Others say there “probably” is, and yet others say that "it's possible", but the evidence is not conclusive.

References:
Bohigian, G.M. “An Ancient Eye Test – Using the Stars” https://www.academia.edu/4982885/An_Ancient_Eye_Test_Using_the_Stars_GM_Bohigian_Surv_Ophth_08_53_5_
Kaler, J. “Alcor” https://stars.astro.illinois.edu/sow/alcor.html
Kaler, J. “Mizar” https://stars.astro.illinois.edu/sow/mizar.html