The Star-Formerly-Known-as-Pole

Snaking along the long body of Draco the Dragon from his head, which we examined in last week’s post, we eventually come to a fairly faint naked-eye star opposite the bowl of the Little Dipper. This is the notable star Thuban.

The name Thuban (THOO-bahn) comes from the Arabic for serpent. Because of the star’s importance to ancient cultures, however, it has born many other names: “Judge of Heaven,” “Proclaimer of Light,” “Crown of Heaven,” and “High One of the Enclosure of Life,” to name a few.

Looking north to Draco and the Dippers
Star maps created with Your Sky

Thuban’s ancient significance stemmed from the fact that it was once the North Star, marking the position of the North Celestial Pole. The North Celestial Pole is the imaginary fixed point in the sky that the Earth's axis would intersect, were it extended from the North Pole into space. The North Star, because it marks geographic north, has been important throughout the ages for navigation.

We are accustomed to the star Polaris, which marks the end of the handle of the Little Dipper, being our North Star. It seems odd at first to think that another star might have been the North Star or Pole Star long ago. In fact, Polaris won’t continue to be our North Star forever. This phenomenon is caused by precession, the change in the alignment of Earth’s axis.

Because the spinning Earth wobbles slowly over time, our axis doesn’t always point at the same spot in the sky. This wobble is caused by gravitational tugging by both the Sun and the Moon. The result is that the position of the North Celestial Pole--where Earth’s axis points--moves over time against the backdrop of the stars, completing a circle in about 26,000 years. The North Celestial Pole will pass closest to Polaris around the year 2100, after which it will begin to move away from it.

The good news is, if we wait just 26,000 years, Polaris will be our North Star once more. What goes around, comes around.

Astronomy Essential: Space is extremely cold.

Intergalactic space, the regions of space that lie between galaxies, is an inhospitable minus 455 degrees Fahrenheit.

Compare that to absolute zero, the coldest theoretical temperature, which is minus 459.67 degrees Fahrenheit, not much colder.

The Sleeping Dragon

Well placed in the northern sky during spring, the constellation of Draco the Dragon winds sinuously between the Big and Little Dippers in the constellations of, respectively, Ursa Major and Ursa Minor. Although the Dippers are among the most-viewed asterisms (recognizable star patterns) in the night sky, Draco often goes unnoticed.

Dare we wake the sleeping dragon?

Draco in J. Middleton’s 1842 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

1) About an hour after your local sunset time, face north. 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 left shoulder to the west, and you’ll be facing approximately north.

Looking north to the Dippers and Draco
Star maps created with Your Sky

2) Locate the Big Dipper in Ursa Major and the Little Dipper in Ursa Minor. Midway between the bowl of the Little Dipper and the very bright star to the east, Vega in the constellation Lyra, is a quadrilateral of fainter stars. This is the asterism known as the Lozenge, and it marks the head of Draco (DRAY-koh).

3) The brightest of the four corner stars of the Lozenge is the orange giant Eltanin. Eltanin (ELL-tuh-ninn) is from the Arabic for serpent. Moving counterclockwise around the Lozenge from Eltanin, we next come to the yellow supergiant star Rastaban. Rastaban (RAH-stuh-bahn) is from the Arabic for serpent’s head.

The Lozenge asterism in Draco

The third corner star in our roundabout has no traditional name, so we call it Nu (NOO) for its star catalog designation. Nu is a binary system, two stars— in this particular case, both white stars— in orbit around each other. With the naked eye we see their combined light as one star. If you have binoculars, use them to look at Nu. You should be able to spot the nearly identical pair.

The orange giant Grumium completes our dragon’s head. Grumium (GROOM-ee-yum) is from the Latin for snout.

The Lozenge asterism was so-named not because it resembles a cough drop, but rather because a lozenge is a diamond-shaped figure. Granted, the misshapen boundaries of this dragon’s head can only be described as a diamond in the rough.

Astronomy Essential: A star's visible color depends on its surface temperature.

Although star color is sometimes subtle, we can observe it, especially when we’re comparing two stars of different colors. Stars that look red, orange, or coppery are at the cool end of the stellar temperature range. White, yellow-white, and yellow stars are in the middle of the range. Our Sun is a yellow star.

The stars at the hot end of the temperature range appear blue and blue-white, the former being the hottest. The average temperature of a blue star ranges between 56,000 and 87,000 degrees Fahrenheit. Now that’s hot.

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.

As the Crow Flies

More so than any other star pattern, when I see the defining asterism (recognizable star pattern) of Corvus the Crow flying high in the night sky after sunset, I know that it’s spring.

Corvus (KORR-vuss) is the Latin word for crow or raven. Although it’s hard for us to imagine a crow among the sparse scattering of stars in Corvus, we can see in classical star atlases how the ancients imagined the raucous bird: bending over to peck at Hydra the Water Snake, the constellation that winds below Corvus.

Corvus from Johann Bode's 1801 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

Let’s go outside and stretch our wings.

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.

Star maps created with Your Sky

2) You’ll find the constellation of Corvus low in the southern sky, just southwest of the brilliant blue-white star Spica (SPY-kuh) in the constellation Virgo the Maiden. If you don’t know how to locate Spica, read this post.

The five brightest stars in Corvus form the small but distinctive asterism known as the Sail. It looks a bit like the wedge-shaped sail of a Chinese junk, with a bit of mast extending below.

Chinese Junk
Image by Wibean

3) The brightest of the four stars that make up the sail shape is the blue-white giant Gienah. Gienah (JENN-uh) is from the Arabic for wing. Moving counterclockwise around the sail shape, we next come to Algorab. Algorab (ALL-gorr-abb) is from the Arabic for raven. Algorab is a binary system, two stars in orbit around each other, although with the naked eye we see their combined light as one star. Algorab’s component stars are a bright white star and a dim orange one; you should be able to see both with even a modest-sized telescope.

The stars of the Sail asterism in Corvus the Crow

Next in line is the yellow-white giant star Kraz. The meaning of its name is unknown. Finally, where the mast meets the sail is the orange giant Minkar (MINN-kahr), from the Arabic for beak.

Dimmer than the four sail stars, the yellow-white dwarf Alchiba (ull-kibb-AH) marks the bottom of the mast. Alchiba is from the Arabic for tent, a harkening to an earlier Arab tradition that saw the star pattern of Corvus not as a bird but as a tent, important shelter for a desert-dwelling people.

Astronomy Essential: The distance from the Earth to the Sun is about 93 million miles.

Because the Earth’s orbit is elliptical, not circular, its distance from the Sun varies a bit over the course of Earth’s year-long orbit. The average distance, however, is about 93 million miles or 150 million kilometers.

Astronomers use a mathematically calculated constant based on the distance between Earth and Sun and call it one astronomical unit or 1.0 AU. They use this as a unit of measure between objects in our solar system. One astronomical unit works out to be slightly less than the average distance between Earth and Sun; it is equivalent to 92,955,807 miles or 149,597,870 kilometers.

Due to the long distances involved even within our own solar system, expressing distances of objects in miles or kilometers quickly becomes cumbersome, making the AU a compact and useful unit of measurement. We can say, for example, that today Neptune is 29.68 AU’s from Earth. Otherwise, we’d have to say it’s 2,758,928,346.9 miles or 4,440,064,781.6 kilometers from Earth. Either alternative is quite a mouthful.