As Arcturus moves, school draws closer


By Tom Burns - Stargazing



Arcturus is the centerpiece of the constellation Boötes, the kite-shaped group of five bright stars north of Arcturus. Arcturus is the tail of the kite.

Boötes is not pronounced like you might pronounce the name of a pair of infant foot coverings. It is — at least for me — the most challenging constellation name to pronounce because it has two long o’s in a row. Try saying it a few times fast — B-oh-OH-teez. See what I mean?

First, let’s find Arcturus. Look high in the northern sky for the Big Dipper. The handle will point to the south.

Extend the handle’s curve directly to the south, and you will see a bright star. It’s easy to remember. Just think, “Arc to Arcturus.”

You’ve found the second-brightest star in the skies of the northern hemisphere and the most brilliant star in the sky during the spring and summer months.

It will rise higher in the east as summer progresses. When I was young, I measured the passage of the summer and the inevitable return to school by the relentless motion of Arcturus as it moved inexorably toward the western horizon.

Its name means the “guardian” or “watcher” of the bear, surely the Big Bear (Ursa Major), of which the Big Dipper is a part.

To many ancient Greeks, Boötes was a herdsman or an ox driver. Perhaps he watches the bear to protect his herd of oxen, which must have strayed because they are found nowhere in the sky.

Other writers claim that he is Erichthonius, inventor of the chariot. Another Greek myth ties him to Icarius, who gave humanity the dubious gift of alcoholic beverages.

Perhaps he is Boiotos, son of the sea god Poseidon and the mortal woman Melanippe. As a baby, Boiotos was cared for and fed by a cow. Yes, that’s right — fresh milk straight out of the spigot, as it were. Later, he may have turned his early affection for bovines into a profession and become an ox herder.

In modern times, many stargazers think of Boötes as the constellation containing Arcturus and for little else.

That association was set in cement by the first century CE. The Greek poet Eratosthenes identified the entire constellation and the brightest star within its confines with the Greek mythological figure named Arcas.

Most mythological sources identify Arcas as the constellation Ursa Minor. Eratosthenes associates Arcas with Arcturus and Boötes. Hyginus, an earlier Roman poet from the first century BCE, makes a similar connection.

Alert! What follows is the unexpurgated version of the Arcas myth. Stop reading if you are offended by reflections of our barely civilized ancient past.

According to Hyginus, Jupiter, the king of the gods, seduced Callisto, an innocent mortal woman. The boy Arcas was born out of their union.

Jupiter loved to wander in disguise among mortals and test their hospitality. One day, the god wandered into a section of Arcadia ruled by the despotic and cruel King Lycaon.

Lycaon wanted to confirm his suspicion that his visitor was an immortal god. So he chopped up the baby Arcas and served Jupiter’s son to him for dinner.

But it’s difficult to fool the king of the gods. According to Hyginus, he was so “disgusted by the king’s inhumanity” that he violently overturned the table, destroyed the king’s castle with a thunderbolt, and turned Lycaon into a wolf.

Jupiter put Arcas back together and promptly abandoned him by giving him to a goatherd to raise.

Despite his difficult babyhood, Arcas grew to young manhood and developed great prowess as a hunter.

At this point, the two versions of the story part company.

Hyginus writes that Jupiter had previously turned Callisto into a bear. Because he grew up far from his mother, Arcas was unaware of the transformation.

One fateful day, Arcas encountered that bear (his mother, remember) in the forest and chased after her, “intent on killing her.”

In a panic, Callisto ran into the sacred temple of Lycaean Jupiter.

Under the laws of Arcadia, anyone entering that temple incurred the death penalty. However, Jupiter took pity on the pair and lifted them into the sky.

Callisto became the constellation Ursa Major, the Big Bear. Jupiter transformed Arcas into Boötes, which Hyginus calls Arctophylax, the Bear Keeper.

Eratosthenes omits Jupiter’s transformation of Callisto into a bear. Instead, he conjures up a romantic relationship between Callisto and her son, Arcas.

The writer hastily explains that the two were unaware of their blood relationship. However, he does not explain how they met or fell in love.

They entered the temple and were married. As in the other story, the inhabitants sentenced the pair to death “according to the law.” What law they broke is unclear in this version. Was it their entry into the temple? Was it their incestuous relationship?

In any case, Zeus (the Greek name for Jupiter) took pity on them because of his previous encounter with Callisto. He lifted them into the sky as Boötes and Ursa Major.

We see Arcas following the bear across the sky, perhaps as he chases it into the temple. He is neither the “guardian” nor the “keeper” of the bear but its hunter.

In 1690, the stellar cartographer Johannes Hevelius snatched a few faint stars beneath the tail of Ursa Major to give Boötes a pair of hunting dogs he called Canes Venatici. The dogs seem to be nipping at Ursa Major’s long tail.

I never realized the significance of Canes Venatici, which Hevelius depicts as two sleek greyhounds, until I hiked in West Virginia more than a few years ago. Hunters tracked and killed bears for a state-offered cash bounty in those days.

I saw a hunter training a pack of dogs to chase a bear into a tree, where the hunter could quickly dispatch it with a high-powered handgun.

Arcas’s star, Arcturus, is bright enough to see with the unaided eye during daylight if you look around sunset and know precisely where to look.

It was the first star seen in broad daylight with a telescope. The French mathematician and astronomer Jean-Baptiste Morin did so in 1635, when the telescope was still in its infancy.

Arcturus is the fourth-brightest star in the nighttime sky after Canopus in the constellation Carina and Alpha Centauri, both of which we cannot see from our latitude, and Sirius, which we can see.

Arcturus’s brightness results in part from its proximity to us at only 36.7 light-years (217 trillion miles) away. Arcturus is also brilliant because it is in the beginnings of its death throes, its orange-giant phase.

The star has used up most of the hydrogen in its core, deep within the star, where hydrogen is fused into helium to make the hydrogen-bomb reaction that fuels a star. It has thus begun to combine the hydrogen in a shell around the core.

It has begun fusing helium into carbon and oxygen inside its core. As a result, it has swollen to an enormous size, around 50 times the sun’s diameter.

If we took the sun out of our solar system and replaced it with Arcturus, the star would extend about two-thirds of the way to the orbit of Mercury.

That dying process produces a considerable amount of energy in the form of light. Consequently, Arcturus is 170 times brighter than that puny yellow-dwarf star at the center of our solar system.

Over millions of years, Arcturus will swell even further as helium fuses to become even more carbon and oxygen. Arcturus will blimp up to a size that would take its diameter out to around Earth’s orbit.

Eventually, a lot of unburned helium “ash” will surround its core and smother its thermonuclear fire. Arcturus will collapse into a dense, white-dwarf star with approximately the volume of Earth.

Heed well the example of Arcturus, oh ye stargazers. The star is a harbinger of future events affecting your planet. Arcturus started with a mass ever-so-slightly greater than the sun’s mass.

It remains so in its orange-giant phase. It took five billion years for Arcturus to get to its sorry state.

Our sun is about five billion years old. Beware.

No, wait a minute. The sun’s slightly smaller mass will delay the dying process somewhat. Behold Arcturus, and see your parent star, giver of light and life, five or six billion years hence.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.