Exploring long, checkered history of Coma Berenices


By Tom Burns - Stargazing



The bright star Arcturus in the constellation Boötes is rising higher in the east right now. Arcturus is a sure sign that spring is finally here in all its glory, and summer can’t be far ahead. Above Arcturus is one of the strangest sights in the sky. The Coma Berenices Star Cluster is a fuzzy patch five degrees wide that resolves into half-a-dozen or so stars to the sharp-eyed observer.

The simplest pair of binoculars will reveal two dozen or so stars, the brightest of which have a very distinctive “V” shape.

Astronomers call such loose associations of stars asterisms. The stars of an asterism look close together in the sky, but that appearance might simply be a coincidence connected with their positions. When we view them from Earth, they just might happen to be all in the same direction but at varying distances.

On the other hand, some stars are born together in litters, like puppies. They form together in giant clouds of mostly hydrogen gas called nebulas. Such stellar siblings are members of what astronomers term an “open star cluster.”

For a long time, astronomers wondered whether the stars of the Coma Berenices asterism were the constituents of a true open star cluster.

Early astronomers noted the asterism but didn’t think much of it. The grouping remained ignored and unloved by both Messier in his catalog and by the more extensive New General Catalogue. In 1915, Philibert Jacques Melotte finally designated the object Mel 111.

At long last, in 1938, astronomer R J Trumpler concluded that the stars combined to form a true open cluster. He determined that they must be weakly bound together by gravity because they are all traveling together as they hurtle through space. Also, they appeared to be about the same distance from Earth.

One problem remained. Most open clusters are usually very tightly packed. What accounts for the star cluster’s loose scattering of stars?

In that regard, astronomers have plenty of specimens to observe. They have discovered over 1,100 such clusters in our Milky Way galaxy, and they will surely find more. True star clusters often contain hundreds, occasionally thousands, of stars.

Mel 111 is a rare specimen because of its proximity to Earth. Several independent estimates, using several differing distance-determining methods, have established that the cluster is about 280 light-years away.

Its distance makes it the second-nearest open cluster to the Earth. Only the Hyades cluster in Taurus is closer at 153 light-years, and the Hyades is also loosely packed. Thus, the proximity of both clusters partly accounts for their large sizes and stellar scantiness.

However, proximity does not account for Mel 111’s apparent scarcity of stars. Over time, many clusters drift apart because of their loose gravitational connection and differing motions.

Not so with Mel 111. When astronomers look at the advanced age of its individual stars, they conclude that it might be as old as 480 million years. It remains a cluster after all that time despite the forces that eventually tear such family groupings apart.

Remember, star clusters are only loosely bound by mutual gravitational attraction. As they travel through space, they become disrupted by close encounters with other clusters and clouds of gas. Their stars are torn from the cluster and join the less-compact stars of the general Milky Way.

Even more stars are lost from the internal motions of the stars within the cluster. As a star mills around in a cluster, it sometimes passes close to another star. As a result, its velocity dramatically increases. The unfortunate star is thus slingshotted right out of the cluster.

Half a billion years after the stars formed out of their parent nebula, the combination of all those factors leaves the Coma Star Cluster looking as old and bedraggled as it can be. However, despite the odds against it, a few members of that stellar family still cling to each other as they wander like nomads through the Milky Way.

The constellation associated with the fuzzy patch, also called Coma Berenices, has a long and checkered history.

In the east, Chinese astronomers associated some of the constellation’s stars with the grand hierarchy of government. Mel 111 and some adjacent stars became the “Officers of the Imperial Guard.”

In the west, few fuzzy patches get their own names, let alone their own constellations. Coma Berenices (Berenices’ Hair) did, but it took a long time to get there.

During its earliest history, Coma Berenices wasn’t considered a constellation at all. The ancient Greeks hooked the fuzzy patch up with nearby Leo, the Lion. They pictured it as a puff of fur at the end of the lion’s tail.

When it comes to constellation naming, the pattern is fairly clear. Virtually all the constellations are named after great heroes, animal and human, from ancient mythology.

Coma is the only constellation named after a real, historical person, Queen Berenice II of Egypt.

Please note a pet peeve of mine in that regard. The constellation’s name is often mispronounced “Bernice,” as in “Hey, Bernice, get up me a root beer, would ya?” Instead, she is Berenice, pronounced “bear-uh-NEE-kay.”

Any queen named after the Greek goddess of victory deserves to have her name pronounced correctly. Think the same thought as you don your NEE-kay running shoes.

Where was I? Oh, yes.

By 150 CE, Ptolemy, the greatest of the Greek astronomers, refers to the stars as “a nebulous mass, called the lock,” i.e., of hair. Even earlier, around 200 BCE, the Greek scientist and writer Eratosthenes identified the puff of stars as Berenice’s Hair in his famous work on constellation science and mythology called The Catasterisms.

Both astronomers worked in Alexandria, the great seat of ancient scholarship at the mouth of the Nile River in northern Egypt. The Berenice in question must surely be Queen Berenice II of Egypt, who lived during the third century BCE, just before Eratosthenes wrote his book.

By the first century CE, the designation was set forever by the Roman poet Hyginus in his Poetic Astronomy, even though he still considered the star patch as part of Leo.

Around 1500 years after, the stellar grouping finally became a constellation in its own right. In 1551, the Dutch star mapper Gerardus Mercator grabbed a few nearby stars and combined them with the fuzzy patch to form the “official” constellation Coma Berenices. In the process, the lion finally lost its tail.

The real mystery is how the stars came to be associated with the hair of an ancient Egyptian queen in the first place.

Historians know very little about Queen Berenice II. During the third century BCE, she was first the sister and then the wife of the Egyptian pharaoh Ptolemy III Euergetes. (The Pharaohs often married their close relatives to keep the royal bloodline pure. Please do not try this at home.)

Berenice was something of a warrior and horsewoman, distinguishing herself in battle before she married Ptolemy.

According to Hyginus, Ptolemy left his new wife just a few days after their nuptials to make war on Asia. Berenice vowed to the gods that she would cut off her hair if Ptolemy returned safely.

That promise had to be considered a major sacrifice. She was apparently quite attached to her long, amber-colored tresses.

At the pharaoh’s safe return, the now-hairless Berenice placed her hair on the altar of the temple dedicated to her mother, Arisinoe.

The next day, the hair was gone. An ancient souvenir collector had probably stolen Berenices’ hair. Temple robbing and pyramid raiding were growth industries back in those days.

The temple priests clearly had some serious explaining to do to a very angry pharaoh and a very bald queen. Conon of Samos, the court astronomer and mathematician, came up with the life-saving solution.

He pointed to the fuzzy patch at the tail of Leo and told the royal couple that Berenice’s tresses had gone to join the stars.

The story has the ring of truth to it. We can imagine a desperate and fearful Conon raising his eyes toward heaven for inspiration and finding there a glorious patch of light.

“There,” he must have said as he pointed dramatically up to the stars. “There is your hair — raised by the gods to the heavens to honor your sacrifice.” And under his breath, he must have mumbled, “Stars. Sacrifice. Yeah. That sounds good. Let’s go with that.”

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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.