The Starry Crowns – Corona Australis
History and mythology
Even though Corona Australis is dimmer than its northern counterpart, it's still associated with a selection of stories. Yet it didn't have a good start. The ancient Greek poet Aratus (315-240 BCE) considered it part of Sagittarius. He also commented that “its jewels [are] not so fair as Ariadne's in the northern sphere.” (The northern crown is Ariadne's marriage crown.) Nonetheless a later tradition identified it as the crown (or wreath) with which Bacchus honored his mother Semele after rescuing her from the Underworld.
Three hundred years after Aratus, Ptolemy (90-168 CE) described the semi-circle of stars as a constellation, naming it Stephanos Notios (southern wreath). Star atlases into the 19th century often pictured it as a wreath rather than a crown, as for example, Johann Bode's Uranographia (1801).
In another story, the constellation was neither wreath nor crown, but rather the wheel of Ixion. Ixion, King of the Lapiths, was a guest of Zeus in Olympus, but he was smitten with Hera, Zeus's wife. The god decided to put Ixion's honor to the test. Ixion failed the test, and he didn't just get a bad report card. He was tied to a fiery perpetually-spinning wheel – Corona Australis.
Not surprisingly, the semi-circle of Corona Australis features widely in the lore of southern hemisphere indigenous peoples. For example, to some it represented people sitting around a campfire, and to others a boomerang thrown by the Dreaming ancestor Totyarguil (the star Altair).
Corona Australis contains twenty-one stars visible to the unaided eye, but no bright ones. Even the two brightest stars, Alpha Coronae Australis (α CrA) and Beta Coronae Australis (β CrA), are fourth magnitude stars. The others are fourth and fifth magnitude. (The higher the magnitude, the dimmer the star appears.)
Only one of the stars in Corona Australis has a name – α CrA is also called Alfecca Meridiana. It means Southern Alphekka, borrowed from one of the many names of its northern counterpart. The star doesn't seem very bright, but in reality it's over twice the size of the Sun, and thirty times brighter. But it's also 130 light years away.
Alfecca and α CrB look equally bright to an earthly observer. However α CrB is an orange giant over five hundred light years away. It's about twenty-five times brighter than α CrA and over seven hundred times more luminous than the Sun.
We can find examples in the crown of the two basic kinds of double star: binaries and optical doubles. Epsilon Coronae Australis (ε CrA) looks like a single variable star because its light varies in intensity. However it's a binary star, meaning it's two stars orbiting each other. In fact, this pair orbits each other so closely that they share an atmosphere, making it a contact binary. In addition, they orbit in our line of sight so that during an orbit, each star comes between us and its companion. This is why the light that we receive is variable.
In contrast to ε CrA, Eta1 and Eta2 Coronae Australis are an optical double. They seem to be close together. However, it's a chance pairing when viewed from Earth. They're a few hundred light years apart.
Deep sky objects
The Corona Australis Molecular Cloud is one of the nearest stellar nurseries to us. It's a dark nebula, so dusty that its background stars are barely visible. But it's not completely dark. Tucked away in the dust are hot young stars giving off UV radiation and creating bright reflection nebulae. One such star is R Coronae Australis, a blue-white variable star. It's hiding inside nebula NGC 6729 which brightens and darkens as the star's output varies.
The Coronet cluster is a small star cluster comprised mainly of young stars. It's over three times closer to us than the better known Orion nebula, making it useful for studying the development of stars and protoplanetary disks. These are the debris disks around young stars from which planets form.
RU Coronae Australis is old and unstable. After using up its hydrogen fuel, it's been expanding and contracting, making its light output variable. It's also been sloughing off its outer layers, and the material has formed the somewhat rounded greenish blue nebula called IC 1297. This kind of nebula looked like the disk of a planet in eighteenth century telescopes, which is why William Herschel called them planetary nebulae.
There are few galaxies in Corona Australis, so NGC 6768 is a rarity. But it's unusual for a more dramatic reason. It's a two-for-the-price-of-one galaxy, an elliptical galaxy and a lenticular galaxy merging. A lenticular galaxy is, in its structure, intermediate between an elliptical and a spiral galaxy.
Note: See the link below this article to find out about the other crown Corona Borealis.
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