Astronomy: The Keeper of Heaven, Part Two


Tom Burns - Stargazing



Last week, we discussed the myth and folklore associated with the star Arcturus in the constellation Bootes, the Herdsman.

It’s easy to see why it was so important to our ancient forebears. It is, after all, the third brightest star in the nighttime sky. It follows Ursa Major, the Larger Bear across the night sky. Thus, to the ancient Greeks, Arcturus was the Guardian of the Bear.”

More importantly, it is the nearest bright star to Polaris, the North Star, which does not seem to move from moment to moment or night to night. Arcturus seems to endlessly spin around Polaris and thus hold it in place. To the Chinese, Arcturus was thus the “Keeper of Heaven.”

Look for it low in the east just after dark. Find the Big Dipper in the north and follow the handle of the Dipper to the east. “Arc to Arcturus,” as the old stargazing maxim goes.

Ancient awe eventually turned into scientific fascination. For centuries, Arcturus in 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 or more miles in diameter, Arcturus is at least 25 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 collapse to a white dwarf and die, sending its outer shell hurtling outward 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 Fahrenheit compared to our sun’s “yellow-hot” 10,000 degrees. Even so, the very sun-like star produces more than 100 times more light than the sun. Such is the fate of old stars. Their temperature goes down, but for a brief time, they shine with incredible brilliance in their decrepitude before they collapse entirely.

Since Arcturus is relatively close to us as stars go, scientists can actually measure the heat energy 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. At 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, albeit indirectly, 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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Tom Burns

Stargazing

Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.