Thursday, June 11, 2009

The Heavenly Herdsman

The name Bootes (boh-OH-teez) is believed to derive from ancient Greek words for ox and driver. Translation into other languages rendered ox-driver as herdsman, and so we’ve come to know this prominent spring constellation as Bootes the Herdsman.

You won’t find the Herdsman’s oxen in the sky. He was, however, associated with the nearby Big Dipper, what the Greeks called “the wagon.” It is this wagon that was reportedly being pulled by Bootes’ celestial oxen.

In a number of classical star atlases, Bootes is depicted as holding the leashes of a pair of hunting dogs: the constellation known as Canes Venatici (KAY-neez vee-NATT-uh-sigh). The hunting dogs are straining at the end of their leashes as they pursue nearby Ursa Major (ER-suh), the Big Bear. The Big Dipper is the central asterism (recognizable star pattern) in the constellation of Ursa Major.

Bootes and the Hunting Dogs in John Flamsteed's 1729 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

During spring and summer, when Bootes is prominent in the sky, you’ll notice that he and his dogs, well, dog the Big Bear across the sky as it circles the North Star counterclockwise. The Big Bear is one of the circumpolar constellations, which means it circles the North Celestial Pole, the imaginary fixed point in the sky that the Earth's axis would intersect, were it extended northward. The North Star, Polaris, just happens to lie at the North Celestial Pole, which is why the circumpolar constellations appear to circle the North Star.

Let’s follow the herd.

1) About an hour after your local sunset time, face south. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

Arc to Arcturus
Star maps created with
Your Sky

2) Tilt your head all the way back and look up at the zenith, the point directly overhead. Locate the Big Dipper, a little north of the zenith. Following the curve of its handle, arc to Arcturus, the brightest star in Bootes.

Big and brilliant, the orange giant star Arcturus is 25 times the diameter of our Sun and 113 times as luminous. Arcturus is Greek for guardian of the bear, a reference to the star’s ancient association with— and apparent trailing of— neighboring Ursa Major. Can you discern its golden or copper hue?

3) Now that you’ve found Arcturus, you can trace out one of my favorite asterisms: the Ice Cream Cone. I’ve a profound weakness for ice cream, so locating this delicious asterism is a sort of guilt-free indulgence.

Ice Cream Cone asterism in Bootes

Most of the bright stars in Bootes form the Ice Cream Cone. Arcturus is the bottom of the pointed cone. Moving north from the point, you’ll come to two stars that form the top of the cone. The one on the left (east) is the orange giant Izar, the second brightest star in Bootes. Izar (EYE-zahr) is from the Arabic for loin cloth. The one on the right (west) has no traditional name, so we call it Rho (ROE) for its star catalog designation. Like Arcturus and Izar, Rho is an orange giant star.

4) Continuing north, we are rewarded with a mound of sweet, cold confection bounded by the stars Delta, Nekkar, and Seginus, moving from east to west. Delta is the star catalog designation for this yellow giant with no traditional name. Nekkar (NECK-ahr) is from the Arabic for ox-driver. Nekkar is another yellow giant star.

Did you ever play the group game “Telephone” when you were a kid? The first kid in a line or circle of kids would whisper a phrase to the kid next to him. That kid would whisper it to the next kid, and so on. The last kid would say out loud the phrase she’d heard. By the time it got to the end of the line, the phrase was invariably garbled, sometimes beyond recognition.

That’s sort of what happened with our third ice-cream-mound star, the white giant Seginus. Believe it or not, the name Seginus (segg-EEN-uss) started out as the Greek name Bootes! It was first mangled in translation by the Arabs. Then the Arabic wrong name was corrupted again by the Romans into a Latinized form. So the name that filtered down into modern times, Seginus, is pretty much meaningless.

Except of course to us, because we know it’s just Telephonese for ox-driver.

Astronomy Essential: Hydrogen is the most abundant element in the universe.

Hydrogen is estimated to make up a whopping 75% of the visible matter in the universe.

Hydrogen is the simplest chemical element. A hydrogen atom is composed of just one proton at the nucleus (core) and one electron in orbit around the nucleus. A proton is a particle with a positive electrical charge, while an electron is a particle with a negative electrical charge.

Stable hydrogen nuclei are believed to have formed only three minutes after the Big Bang. It took another 700,000 years or so for the nuclei to collect their electrons and become stable atoms. Hydrogen was the most abundant element in the universe back then, too.

Hydrogen is the fuel that powers stars. Hydrogen burns into helium in the cores of stars, and this nuclear reaction produces energy in the form of heat and light.

After oxygen and carbon, hydrogen is the third most common element in the human body. It is a primary component of water, and our bodies are composed of more than 50% water. Without hydrogen, human life as we know it could not exist.