The Starry Crowns – Corona Borealis
History and mythology
Both crowns are constellations of ancient lineage, and in the second century Ptolemy (90-168 AD) listed them in his Almagest.
The constellation that we call Corona Borealis has had other names in classical times, including Ariadne's Crown, as in the third century BC Argonauticae of Apollonius Rhodius. This refers to the best known of the stories surrounding Corona Borealis, that of Theseus, Ariadne and the Minotaur – a tale of heroism, love and betrayal.
The Minotaur was a monstrous creature with a bull's head and a man's body. The king of Crete hid him away in a labyrinth from which there was no escape. When the Cretans had defeated the Athenians in battle many years before, they demanded a terrible tribute. Every seven years Athens had to send youths and maidens to sacrifice to the Minotaur. But one tribute year Theseus – handsome hero and wrestling champion – vowed to kill the Minotaur and end this scourge.
Ariadne, daughter of the king of Crete, fell in love with Theseus. She gave him a spool of thread to unwind in order to find his way out of the labyrinth. He killed the Minotaur, followed the thread, and ran off with the princess. But then he abandoned her on the island of Naxos. Happily, the god Dionysos found her there, and was smitten. He married her and threw her marriage crown into the heavens in celebration. The jewels changed to stars. Gemma, Latin for jewel, is one name for the bright star Alpha Coronae Borealis.
However North American indigenous peoples have seen the constellation variously as a bear's den, an eagle's nest, a camp circle, or the smokehole over a fireplace. The Welsh name for the Corona Borealis is Caer Arianrhod, for it represents the castle of Lady Arianrhod the Celtic moon goddess. In Australian aboriginal astronomy, the constellation is simply Womera (the Boomerang) which it resembles.
Stars and planets
Alpha Coronae Borealis
Alpha Coronae Borealis (α CrB, for short) is a second magnitude star, but the other stars in the semi-circle are much dimmer fourth magnitude stars. The Arabic name for the constellation was Alphekka. which at some point became a name for α CrB, and later, as Alphekka Meridiana, it was adopted for the star Alpha Corona Australis.
Besides its brightness, α CrB has two interesting features. The first is that it's not a single star, but a binary system with a blue-white primary star nearly three times as massive as the Sun. The second interesting feature is that α CrB A seems to have a dust disk extending to about 60 astronomical units. (That's the distance from the Sun to the far edge of the Kuiper Belt.) If so, it could have a planetary system.
The secondary star of the binary, α CrB B, is similar to the Sun, but much younger. It's also hotter and rotates more quickly than the Sun. The two stars of α CrB orbit each other in such a way that one comes between us and its companion every few weeks. It's the type of system known as an eclipsing binary.
It wouldn't be surprising to find planets orbiting α CrB. As of June 2015 we know of five stars in Corona Borealis that have planets. All five planets are Jupiter-sized, including a hot Jupiter that orbits its star every three days.
T and R Coronae Borealis
Corona Borealis boasts two rare types of variable star. T Coronae Borealis (T CrB), also known as the Blaze Star, is a type of cataclysmic variable known as recurrent nova. If “cataclysmic variable” sounds explosive, that's because it is. The star can brighten suddenly by eight magnitudes following a nuclear reaction and explosion.
T CrB is a binary star with a red giant primary and a white dwarf secondary which orbit each other in about eight months. A white dwarf is the dense remains of medium-sized star that ran out of fuel and collapsed. If it's close enough to steal material from a companion star, it can get enough mass to set off a thermonuclear explosion on is surface. That's called a nova and can happen repeatedly, unlike a supernova which blows the star apart.
As for yellow supergiant R Coronae Borealis (R CrB), you might think of it as a nova going into reverse. R CrB can suddenly dim by up to nine magnitudes before brightening again. Astronomers think that the star ejects carbon dust. If the carbon clouds are in our line-of-sight, we see R CrB dim. When the stellar winds blow the clouds away, we see it brighten again. The Hubble Space Telescope has found extensive dust clouds around the star.
Deep sky objects
Corona Borealis isn't rich in deep sky objects. Its major one is so distant that it's viewing for fairly large telescopes. It's the Corona Borealis Supercluster, which includes at least seven galaxy clusters. The most massive of them is Abell 2065, more than a billion light years from Earth and containing over 400 galaxies.
And there is also an intriguing galaxy cluster that's nearly four billion light years away. Called RX J1532.9+3021, at its center is a giant elliptical galaxy with one of the most powerful supermassive black holes ever discovered.
Note: See the link below this article to find out about the other crown Corona Australis.
You Should Also Read:
ABC of Astronomy – B is for Bok Globule
The Starry Crowns - Corona Australis
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