Corona Borealis
Published on Jul 7, 2007 at 1:38 pm.
2 Comments.
Filed under constellations, sky lore.
The constellation Corona Borealis is located between Vega and Arcturus. It is currently high in the sky at sunset. The image that I’ve loaded is for either much later as it is setting, or this fall when it is low in the western sky at sunset.
Corona Borealis is the “Northern Crown.” The ancient Greeks only know of the one crown, so it was the “Wreath” to them. It is tiara shape formed of stars. According to mythology, Corona Borealis was the crown worn by Ariadne, daughter of Minos. Those that recall mythology will remember that King Minos of Crete kept the Minotaur. Minos’ son Androgeos had been killed in battle with the Athenians. Crete won the war, though, and Minos imposed a heavy penalty on Athens: each year, seven young men and women were to sail from Athens to Crete to be used as food for the Minotaur. The third year of this punishment, though, Theseus, son of King Aegeus, convinced his father that he could defeat the Minotaur if he were allowed to be part of the contingent sent to Crete. Aegeus, after much argument, finally agreed, with the stipulation that the ship carrying Theseus back to Athens fly a white sail instead of the traditional black one to signify that Theseus has survived the fight with the Minotaur. Well, Theseus did win the battle, killing the Minotaur. He also fell in love with Ariadne. She had helped him out by providing him with a spool of yarn that he played out behind himself as he walked through the labyrinth containing the Minotaur. This allowed him to find his way back out after dispatching the monster. Ariadne was madly in love with Theseus, and went with him on the trip back to Athens to be his wife. However, along the way, they stopped at Naxos, the island belonging to Bacchus, god of wine. Bacchus was stricken with Ariadne’s beauty, and he ordered Theseus to sail away leaving Ariadne behind, and without even saying goodbye to her. Theseus, unwilling to risk the lives of his companions fighting with a god for Ariadne, did as he was told. But, he was still very much in love with Ariadne, and he was so despondent that he forgot to order a white sail to be hoisted upon their return. King Aegeus, seeing the black sail, thought that his son was dead. In despair, he flung himself off a cliff into the sea, killing himself. That sea is now known as the Aegean Sea. Meanwhile, Ariadne was in tears over her lover leaving her during the middle of the night. Bacchus consoled her, telling her that Theseus wasn’t worth her time, since he had left her. Bacchus wanted Ariadne to marry him. But, Ariadne was in no mood to marry another mortal man, and she was wary of Bacchus, wanting him to prove to her that he was a god before she married him. So, he took her crown and threw it upwards into the sky to become the group of stars that is now the Northern Crown, Corona Borealis. The brightest star of Corona Borealis is Gemma, the gem star. Seeing that he was a god after all, Ariadne agreed to marry Bacchus.
But, not all people have seen this circlet of stars as a crown. To the Chinese, the this little group of stars was a money belt. The Chinese had coins with holes in them, and they tied a cord around their waist going through the coins. The stars were the coins on the belt.
The Arabs also did not see this as a crown. To them, this was a beggar’s bowl. The brightest star in the bowl was called Alphecca, “the bright dish.” Actually, that was the name that I learned the star as so many years ago. But, I knew the constellation as Corona Borealis, the northern crown. It was always hard to remember Alphecca, since in my mind I kept thinking “alfalfa” whenever I said Alphecca. Then, I looked on another star chart, and the star that I had learned from my first star charts as Alphecca was labeled Gemma. That made much more sense in a crown. This illustrates the problem with proper names for stars. The names are often Arabic, and the constellations are mostly Roman (most of those stolen from the Greeks). These stars are known by many names: Arabic, Greek, Latin, Chinese, and many others. It was to alleviate this confusion that Johannes Bayer created a designation system for stars. In the Bayer designation, this brightest star in Corona Borealis, known by the proper names Alphecca and Gemma, is called Alpha Coronae Borealis, written as “α CrB.”
But, there is another interesting star in Corona Borealis. That is the star R CrB. The star R Coronae Borealis is a variable star. It is classified as a type of eruptive variable, but unlike most other eruptive variables R CrB stars don’t get brighter with their eruptions. Instead, at irregular intervals, R CrB stars (of which the star R CrB is the prototype) dim by several magnitudes. The degree of dimming and its duration are irregular. Sometimes they stay dim for some time, and sometimes they begin to brighten only to dim again.
After careful study, astronomers have finally decided that the cause of R CrB stars’ dimming is dust clouds forming around them. These dust clouds are very carbon rich. R CrB stars are supergiant stars in the last stages of their lives. Dredge-up convection has brought carbon from the deep interior of the star to near its surface. Carbon is produced in the nuclear fusion that occurs a couple steps after the star runs out of hydrogen to fuse in its core and leaves the main sequence. Mass is ejected from the star, and as the material moves outward, it eventually reaches a distance that permits the carbon to condense into molecules (probably some of these are crystalline — diamonds!). These dust clouds then sometimes move into our line of sight, causing the star to appear dimmer. Eventually light pressure pushes the dust clouds away, and the star appears its normal brightness again, as seen from Earth.
I’ve watched R CrB go through a couple of these dimming events, and I’ve even collected data on it. The star is dim enough to normally be right at the threshold of visibility with unaided eyes, but it is easy to see in binoculars. And, it can even be tracked as it is dimming in binoculars unless it goes into a particularly deep eruption. So, you might want to go look for it sometime. I have it marked in the chart above, along with the star Gemma (or Alphecca, as I first learned it and marked it on the chart).
-Astroprof
Star charts produced using Starry Night Pro
A Ler…-- Rastos de Luz on July 9, 2007 at 8:52 am: 1
[…] “Corona Borealis“, no Astroprof’s Page. Também neste blog “Aphelion“; […]
Roxane on December 29, 2007 at 1:07 pm: 2
Wonderful news to learn the meaning and history to “Gemma” from a celestial point of view. Used as a boat name and currently becomming a popular female name stimulated my interest. This sight has given me a new interest in the greek methology and the night time sky, especially while sailing.