Truth about Big Dipper


By Tom Burns - Stargazing



Most of us are stuck in our dwellings during these troubled days. We emerge to walk the dog or engage in socially distant conversations with our neighbors. If we are lucky, we can pursue our work from home.

As a teacher, I am one of the lucky ones. I can teach my students at Ohio Wesleyan remotely if I can just puzzle out the complexities of “synchronous communication,” a phrase I was unfamiliar with just 10 days ago but now fills my days.

I feel most constrained at night. I miss the clear, dark skies of rural Ohio. My star-deprived neighborhood is an inadequate replacement. Nevertheless, just after dark, out into my light-polluted backyard I go to relive my first, youthful experiences of the sky.

And there it is — the good, old Dipper high in the north.

Most people know the Big Dipper when they see it. When I ask people what constellation they learned first, they almost always mention the Big Dipper.

What most people don’t know is that the Dipper is not a constellation at all. The Big Dipper is part of a larger constellation called Ursa Major, the Big Bear. Mere collections of stars within a constellation are technically called asterisms.

During springtime, the Dipper rises quite high in the sky, almost to the zenith, and the seven dipper stars are visible to the unaided eye — even from our light-drenched urban skies. In my youth, many were the warm spring nights I saw it there.

The handle of the dipper is the bear’s tail. The rough square that forms the dipper is its prominent posterior. Fainter stars form its head and the three legs that stretch below.

One of my first astronomical experiences was seeing the faint star that sits quite close to the bend in the Dipper’s handle. The bright star at the bend of the handle is called Mizar. Its fainter companion is called Alcor.

According to the latest measurements from the Gaia spacecraft, the two stars are very close by astronomical standards at only about 1/3 of a light-year away from each other. Yes, that’s right. A distance of 1.8 trillion miles is considered close by stellar standards.

Alcor and Mizar are traveling through space together. Although astronomers aren’t quite sure yet, they might be gravitationally bound. If they are, Alcor slowly orbits Mizar over a period that might take thousands of years.

When I finally got a telescope when I was 12, I trained it on the cosmic duo. I was pleasantly surprised to discover that Mizar was, itself, two stars of about equal brightness. Mizar A and B, as they are called, are definitely bound together by gravity. As a result, they orbit each other in a cosmic dance called a binary star.

As a kid, I was a budding astronerd. I read everything about the science of the stars that I could lay my hands on.

When I reached college, a surprise awaited. From ancient times, people had told stories about star groupings like the Big Dipper. Those tales revealed much about the way we humans were a long time ago. But because they resonated so deeply in my heart, I soon realized that they told me much about who I truly was and how I truly felt about the world around me. In short, I understood at last what a lonely kid I was.

In that regard, I will never forget the moment that I first read these lines from Homer’s great epic, The Odyssey:

“… the Great Bear

By others called the Wain, which wheeling round,

Looks ever toward Orion and alone

Dips not into the waters of the deep.”

Poor Ursa. Poor Odysseus. Of all the Greeks warriors who survived the Trojan War, only he was forced to wander the unknown sea for years before he could return home.

Ursa Major expresses that lonely life because from our latitudes it must forever circle the pole star, never setting below the horizon. Most of the other constellations rise and set into the horizon, which for a country like Greece with its long coastlines, means that they set into the water, the “great deep.”

Like the bear in his lonely wanderings, Odysseus never seems to be able to return to the comfort of home, hearth and family.

Since they would have recognized the myths associated with Ursa Major, the Greeks and Romans who read the lines above would have felt that loneliness immediately. The best-known story illustrates what must have been the most important rule for the ancients — don’t fool around with the gods.

Calisto didn’t listen. She attracted the amorous attentions of Zeus, the head honcho on Mount Olympus. His wife, Hera, was jealous, so she changed Calisto into a fat bear, constantly pursued by hunters.

One of those hunters was Arcas, the brave and powerful son of Calisto. He pursued the great bear with unflagging ardor. He never knew that the great bear was his mysteriously missing mother.

Finally, Arcas found the bear, and now the stage was set for one of the most dramatic and poignant moments in Greek mythology. Calisto spied her son. Her joy was so unbounded that she rushed to embrace him. Arcas, in fear of his life, raised his mighty bow and was about to kill the woman who had given him birth.

At that moment, Zeus intervened. He turned Arcas into a bear smaller than his mother. His bow fell from his clumsy paws, and Calisto was saved from death. More importantly to the ancients, Arcas was saved from committing inadvertent matricide.

Zeus then grabbed the greater and lesser bears by their tails, stretching them to their extraordinary lengths as he dragged them into the sky as Ursa Major and Ursa Minor.

Hera got the last laugh by conspiring with the god of the sea to put Calisto and her son in the north — in a place where they must ceaselessly circle the North Pole in a never-ending quest for the sweet oblivion of sleep.

The “waters of the deep” could also refer to our galaxy, the Summer Milky Way, which will soon stretch to the east through the rising glory of Cassiopeia and Cygnus.

Because the Big Dipper is so far from the densest part of the Milky Way, we can see through it to other galaxies such as M81 and M82, a spectacular pair near the bear’s head.

Those galaxies are as much as a million times farther away than the familiar stars of our galactic neighborhood, which are visible all around as you stand in the warm fragrance of a spring night. In effect, galaxies are island universes. They exist in profound isolation from each other.

I hope that you will take a moment and look. Like Alcor and Mizar, we humans travel together through the vastness of space. A mere force of nature gravitationally binds the stars. If we humans are lucky, we are linked by bonds of love and care that transcend the space and time that now keep us apart. No cursed coronavirus can triumph over that.

So go outside and look. As you stand there surrounded by all that beauty, think too of the cold winter that has just passed, of the lonely wanderings of Odysseus and Calisto, and of the unending quest of the Great Bear, who will never merge into the gloriously rising splendor of the summer Milky Way.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.