Rise, fall of Arcturus


By Tom Burns - Stargazing



The season we laughingly call spring will soon be over. This year, spring came in like a lion and looks like it will go out like a dead lion. No. Wait. That’s not right.

The old saying about the lion and the lamb refers to the stormy weather of early spring, which eases slowly into the more benign meteorology of summer.

The lion-lamb adage might also have an astronomical connection. As spring begins, Mighty Leo, with his turbulent ways, rises in the east. By the end of spring, the more stable Aries, the Ram, is rising in the morning sky.

I never put much stock in such matters. For stargazers (this one, at least), spring begins when Arcturus rises again in the east in the early evening. Sadly, poor Arcturus used to be one of the most important stars in the heavens, but now few people can find it.

Let’s rectify that unjust situation.

The easiest way to find Arcturus is to start with the Big Dipper, the brightest stars of Ursa Major, the Great Bear. You’ll find the “Big Dip” high in the northeast around 10 p.m. Follow the handle as it arcs around, and keep going until you come to a bright, yellow-orange star in the east. “Arc to Arcturus,” as the old stargazer’s saying goes. You can’t miss it. Arcturus is the brightest star of spring.

Because of its closeness to Ursa Major, the Greeks called it the “Guardian of the Bear.” They believed that its power drove the Great Bear across the sky. In fact, Arcturus is the brightest star in the kite-shaped constellation Bootes (boh-OH-teez), the Herdsman. It may seem odd for him and his brightest star to be herding a bear, but there you go.

You are looking at the fourth brightest star in the sky. The ancient Egyptians worshiped it as a god. It was the Arabian “Keeper of Heaven.” To the Chinese, it was the golden “Palace of the Emperor.” By closely observing its rising and setting, the ancient Greeks used it to set the dates for their annual spring festival. For most folks in the northern hemisphere, it heralded the return of spring, even in places like Egypt where the winter is not nearly so bad as Ohio’s.

Sailors saw its rising as a harbinger of violent storms at sea, which is not surprising since it rises in the year’s most turbulent month. Around 460 BCE, the Greek physician Hippocrates believed that its position in the sky influenced human health. At its rising in the spring, “diseases often prove critical.” One was apt to die at the rising of the star.

A word for the superstitious: I’ve been watching Arcturus rise for over 50 springs, and I feel fine, thank you very much.

For centuries, Arcturus has been the subject of close scientific scrutiny. We now know that it is about 37 light years, or about 220 trillion miles, away from us. At 20 million miles in diameter, Arcturus is 25 or more times wider than our sun. Yet, it contains only about the same amount of “star stuff” as our yellow dwarf star, the sun. Its stellar material is spread very thinly indeed, with only 1/3,000 the density of the sun.

In effect, Arcturus is a harbinger of things to come for our sun. As an “orange-giant” star, it has reached the end of its life. Like Arcturus, the sun will eventually swell to enormous size and engulf its inner planets, perhaps even to the orbit or Earth.

In a few hundred million years, Arcturus will then collapse to a white dwarf and die, sending its outer shell hurtling into space. Our own sun will suffer the same fate, but not so soon: in five or six billion years.

Arcturus is a relatively cool star compared to the sun. It is only “orange-hot” at 7,000 degrees compared to our sun’s “yellow-hot” 10,000 degrees. Since Arcturus is relatively close to us as stars go, scientists can actually measure the heat we receive from it. It isn’t much — only about the amount you’d get from a candle at five miles away.

For a brief moment in 1933, Arcturus reached the pinnacle of its fame. For the opening of the “Century of Progress” Exposition in Chicago, the light from the star was focused through telescopes on a photoelectric cell. The energy thus generated was used to flip the switch that turned on a huge bank of floodlights, and the Chicago Exposition was for the first time ablaze with the light of a distant star.

Since then, it’s been all downhill for Arcturus. Let’s face it. People don’t look at the stars as much as they used to. The very outside lighting used at the Chicago Exposition now illuminates our cities, and the stars have grown dimmer as a result.

Over the long run, the prognosis is bad for Arcturus. It is moving in our general direction at 90 miles per second, and it will reach its closest point to us — at just a few hundredths of a light year closer — in about 4,000 years. However, its arcing path will eventually cause it to move away. In a few million years, its distance from us will cause it to fade from view, and the “Keeper of Heaven” will desert us for a million centuries.

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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.