The Herdsman and the Crown

By Dennis Mammana

May 17, 2018 4 min read

Week of May 20-26, 2018

Midway up in the eastern sky on spring evenings lie two of my favorite stellar groupings. One is the small but quite beautiful constellation known as Corona Borealis, the Northern Crown.

The Crown is not one of the brighter star groupings, and you'd be better off searching for it from a dark-sky location early this week, before the moon becomes too bright. You can find it easily by locating a few brighter stars first.

After dark this week, look midway up in the eastern sky and you'll see a glistening yellowish-orange star named Arcturus. You can find it by following the "arc" of the Big Dipper's handle outward, away from its bowl. Arcturus is the fourth brightest star in the heavens and the most brilliant star north of the celestial equator.

Located not far from the constellations Ursa Major and Ursa Minor — the Great Bear and Little Bear — Arcturus marks the constellation Bootes (pronounced bo-OH-teez), the Herdsman or Bear Driver; it is named after the mythological herdsman who eternally shepherds the polar stars in their daily and annual revolutions around the North Celestial Pole.

Go ahead and search the area for a herdsman, if you like, but anyone who has read my column over the years knows that I prefer to do things the easy way. To me, Bootes looks much more like a horizontal kite, with Arcturus forming its base, where a tail streams nicely off to the south.

The name Arcturus derives from the ancient Greek word "arktouros," meaning "bear guard." It's a red giant star about 25 times larger and about 180 times more powerful than the sun — an example of our star's fate some 5 billion years from now — and lies about 222 trillion miles, or 37 light-years, away.

Just east of Bootes you will see a tiny arc of stars known as Corona Borealis. One story says that this ancient constellation symbolizes a jeweled crown, or wreath, worn by Ariadne, daughter of King Minos of Crete. Another says that it represents a braid of golden twine. Whatever the true story, Corona Borealis is one of the smallest constellations in the heavens, ranking 72 out of 88.

The Crown's brightest star is named Alphecca and is eight times fainter than Arcturus. What's more, it's not actually a single star, as it appears to the eye, but two. Even large telescopes can't distinguish the individual stars that make it up, but astronomers have learned quite a bit about it by studying how its weak stream of light brightens and dims over time.

For example, they have deduced that these two stars eclipse each other as they orbit a common center of gravity every 17.4 days — an eclipsing binary system. The brighter of the pair is a large white star with a temperature around 17,000 degrees Fahrenheit; it appears to be about 2.5 times larger, 50 times more luminous and three times more massive than our sun.

The fainter companion is a star more like the sun — perhaps a bit smaller and cooler — that orbits the primary star at a distance of about 40 million miles.

 View the Herdsman and the Crown after dark this week.
View the Herdsman and the Crown after dark this week.

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