Astronomy: The horse and his rider


As winter turns to spring, now is a great time to check out the Big Dipper and its most famous star. Look high in the west above Polaris, the North Star.

At the bend of the dipper handle is its most celebrated member, Mizar. Like most of the brighter stars in the northern sky, Mizar was named by Arab astronomers. Its name means “girdle.”

Huddling close to Mizar is the somewhat fainter Alcor, which means “brassiere.” No. Wait. My mistake. It probably comes from the Arabic Al-jat, which means “the rider.” The stars together are often called the Horse and His Rider.

In fact, the two stars have been horsing around for a long time. In England, Alcor was called Jack on the Middle Horse, Mizar being the starry steed. Along with the other stars in the handle of the dipper, Mizar is part of the team of horses that push a plow or pull a wagon, both of which the English see as the bowl of the Dipper.

The Romans sometimes used the name Eques Stellula, the Little Starry Horseman for Alcor. The Dutch believed it was Hans Duemken, a legendary wagon driver who spends eternity driving his team of horses across the sky. Again, the stars in the handle form the horses, and the bowl of the Dipper is the wagon.

Because of its proximity to the brighter Mizar, Alcor is the Rodney Dangerfield of the starry sky. It just can’t get no respect. The Arabs sometimes called it Suha, the Forgotten One, and the Chinese called it Foo Sing, Supporting Star.

The Arabs considered Alcor a test of vision, a celestial eye chart, which is odd because Alcor is easy to see with the unaided eye. The Roman military may have done the same. If you could see it, you were in the army. I can just imagine the reluctant recruit: “Star? What star? I don’t see any star.”

According to an ancient Greek story, Alcor started out as Elektra, one of the Pleiades or Seven Sisters. Because they could only see six stars in the cluster, they believed that Elektra had wandered from her sisters, and had been transformed into a fox. She now lives among the stars as the now the much-diminished Alcor, which they called Alopex.

In Indian philosophy, the stars of the dipper represent the Seven Rishis, or wise men. Mizar is one of the rishis, Vashishtha. Alcor is his devoted wife Arundhati. The pair travels inseparably across the sky.

They have thus come to symbolize the great bond that exists between couples in marriage. According to M.K.V. Narayan, it has even become part of the marriage ceremony in some communities.

Mizar is one of the most important stars in scientific history. In 1650, soon after Galileo first used a telescope for astronomy, Giovanni Riccioli used his to discover that Mizar consists of two stars, the first true binary system ever discovered.

True to their penchant for giving stars clever names, modern astronomers call the stars Mizar A and Mizar B. They are very young and thus rich in the hydrogen that fuels their thermonuclear reaction. They are just a hair’s breadth from each other in cosmic terms. Separated by 36 billion miles, they are about 400 times as far from each other as Earth is from the sun.

The stars orbit each other, kind of like two thumbs engaged in a cosmic twiddle, with a single twiddle taking thousands of years.

In 1889, William Pickering discovered that Mizar A has a faint companion that couldn’t be seen in a telescope. The moment was, well, momentous. For the first time, an astronomer had used a spectroscope to discover a binary star system by breaking up the stars light into its component color bands.

The companion orbits Mizar A in only 20.5 days. Mizar B also turns out to be a star system with three components. Thus, the Mizar system is composed of at five stars, orbiting each other in a complex pattern.

Alcor also turns out to be a system with two components. For now, they move together in space, but they are slowly drifting apart. At least for the next 100 million years or so, the total number traveling companions comes to seven.

Many such multiple-star systems have been discovered. Most stars are associated in pairs and even in groups of three or more. If you want to know what the cosmos is really like, take a spin on the teacups ride at your local county fair.

Our own star, the sun, turns out to be a cosmic oddity, a single star floating in majestic isolation, alone in the vastness of space.

Tom Burns


Tom Burns is director of the Perkins Observatory in Delaware.

No posts to display