Golden Star Arcturus Shining Bright for Skywatchers

There is a brilliant star that sparkles with a golden yellow or topaz hue, positioned halfway from the eastern horizon to the point directly overhead (the zenith) as darkness falls this week. It?s Arcturus, and it resides in the constellation of Bo?tes, the Herdsman.

Arcturus ranks as the fourth brightest star in the night sky overall, behind Sirius, Canopus and Alpha Centauri. In older astronomy books, it was ranked sixth behind Vega and Capella, but modern measurements have since shown it to be a trifle brighter than those two stars.

As if it weren't already easy enough to find, there is another way of locating it.

Look at the Big Dipper. Its handle is bent. Imagine extending a curve from the handle; a curve that is readily translated into a smooth arc. Continue that imaginary arc about the length of the Big Dipper and you will ultimately come to Arcturus.

So remember: "Follow the arc to Arcturus." Some folks may add an additional line: "Then speed to Spica," for that same arc, continued past Arcturus for roughly the same distance, will come to the bright bluish star Spica in the constellation of Virgo.

Arcturus is estimated to be roughly 25 times the diameter of the Sun — about 20 million miles. Its luminosity is about 115 times that of the Sun. In his definitive three-volume work "Burnham's Celestial Handbook," Robert Burnham, Jr. points out that with modern infrared recording devices, the heat received from Arcturus can be measured, and is found to "... equal the heat of a single candle at a distance of about 5 miles."

Time travel

It was 77 years ago this month, on May 27, 1933, that Arcturus became the object of a great publicity buildup in Chicago, at the opening of the Century of Progress Exposition. At 9:15 p.m. Central Time that night, the star's light was focused by telescopes on photoelectric cells, and the current generated was used to activate the switch to turn on the floodlights at the exposition grounds.

Arcturus was chosen for this task because Chicago had a great World's Fair in 1893. The best estimates at that time said that the light from Arcturus required 40 years to reach Earth. So a story was built around the fact that the 1933 fair would be opened by light that had started its trip while the 1893 fair was still in progress.

Our more accurate measurements today place the light-time distance of Arcturus at 37 years. So it was really starlight that started toward Earth in 1896 that turned on the lights of the 1933 fair.

Shifty star

Stars are typically said to maintain relatively fixed positions in the heavens. But they all move in respect to our vantage point, some much more than others.

Arcturus appears to shift its "fixed" position in the sky much more rapidly than most of the other bright stars, the sole exception being Alpha Centauri. Edmund Halley was the first to discover this motion back in 1718.

Arcturus appears to whiz through space at a speed of nearly 90 miles per second in the direction of the constellation Virgo. It appears to move toward Virgo by about one-degree (which is about twice the apparent width of a full Moon) over a time span of about 1,500 years. We are thus very fortunate to live at a time when the distance separating Arcturus and our solar system is nearly at its minimum.

The star will continue to approach the Earth for several thousand years more, but then it will pass us as it continues to move toward Virgo and its distance from us will begin to steadily increase. Computations show that in approximately 500,000 years, it will probably have moved out as far as 800 light-years away from us and likely will have faded completely from naked-eye visibility.

Arcturus' home, the constellation Bo?tes, has its own story.

The dots over the second "o" in Bo?tes mean that you should pronounce the vowels separately: Boo-OH-tes, not (as many people say) Boo-ties! It is a Greek word meaning a man who tends to a herd of cows, so why not refer to Bo?tes as the cowboy of the sky?

In the allegorical sky pictures of star atlases of a few hundred years ago, Botes is usually pictured holding a large rod or staff. Bo?tes is supposedly chasing after Ursa Major, the Great Bear. This is why some legends refer to Bo?tes as "The Bear-guard."

On our sky map Bo?tes resembles a narrow kite, but it might also be envisioned as an ice-cream cone. At the bottom of the cone shines Arcturus. Since it has an orange hue, we might say that the cone was holding orange sherbet, and perhaps somebody bit off the bottom of the cone allowing the last little glob (Arcturus) to dribble out.

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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.