By Jim Kaler

Did the makers of the constellations, both ancient and modern, lack some imagination? So many come in pairs or trios: Ursa Major - Ursa Minor (Greater and Smaller Bears), Hydra - Hydrus (Water Serpent, Water Snake), Triangulum- Triangulum Australe (the Triangle and the Southern Triangle), the list expanding even more if we allow asterisms and defunct patterns. Among this set are two of the most exquisite figures of the sky, two ancient royal symbols, Corona Borealis and Corona Australis, the Northern and Southern Crowns, the former the crown of Ariadne, the latter that of Sagittarius, south of which the Southern Crown gracefully curves (the myths wreathed around the two extensive indeed). While Corona Borealis (shortened to "CrB"), lying just east of Bootes, is well known to northerners, the far southern position of Corona Australis (CrA, 40 degrees south of the equator) places it near or below the horizon for those in northern Europe, northern Canada, and Great Britain, sadly rendering it out of sight and largely out of mind.

The luminary of the Northern Crown is a fine second magnitude class A star 75 light years away, one much like Vega, that goes by two popular names: "Alphecca" (from Arabic, alluding to the "broken" nature of the constellation's semi-circle) and from its perfect jewel-like placement, "Gemma." Gemma stands out as a subtle eclipsing double. Every 17.3599 days, a rather sunlike companion much smaller and fainter passes directly in front of the bright member to produce a drop of a tenth of a magnitude, the pair but two-tenths of an Astronomical Unit apart. The companion, a strong X-ray source, is (like the Sun) magnetically active.

Though the brightest stars of the Southern Crown are fainter than those of the Northern, CrA boasts not just one, but two luminaries, at least so far as naked eye observations are concerned. Alpha CrA (from its northern counterpart known as "Alfecca Meridiana"), at mid fourth magnitude, is less than two hundredths of a magnitude brighter than Beta -- if that. Thus paired, they make a nice contrast, Alpha (in parallel with CrB's luminary) a white class A star 130 light years off, Beta a much- more-luminous class K orange giant a bit over 500 light years away.

While the small angular areas of these two constellations do not allow them to hold huge numbers of astronomical objects, they are not without good, even famed, inhabitants. In the Northern Crown, two stars top the list. Among the best-known variables of the sky, and a favorite of both professional and amateur star- watchers, is R Coronae Borealis. Improperly, though affectionately, known as "R Cor Bor," it is the prototype of about 40 similar Galactic class F and G supergiants (plus a bunch more in the Magellanic Clouds). It can sit quietly at sixth magnitude for years, then will suddenly disappear from the naked- eye sky, dropping in brightness to as faint as fourteenth magnitude, after which it may take up to a year to return to normal. Nearly 5000 light years away, R CrB is a rare form of "carbon star" that has been stripped of its outer envelope by means of a wind or other process, the carbon coming from advanced nuclear fusion reactions, the star clearly in its death throes. Every now and then, it will launch a blast of gas toward the observer in which the carbon condenses to solid soot, thus hiding it from view. Exactly where such stars fit into the grand scheme of stellar evolution is argued, and really unknown.

Close on the heels of "R" is "T," a wonderful "recurrent nova" that erupted from 10th magnitude to second in 1866, then in 1946 blasted out to third, in both cases falling back to below naked-eye vision within days. The T CrB events are caused by hydrogen fed from a class M red giant to a massive and dense white dwarf companion. Over a period of only decades, the hydrogen heats to the point of nuclear detonation, which hurls the white dwarf's surface layers outward. After the star settles down, the process begins all over again.

Such stars are candidates for total destruction. Stars like the Sun are supported, kept from contracting, by energy- generating thermonuclear fusion (conversion of hydrogen to helium, or in the case of giants helium to carbon and oxygen). White dwarfs, however, are dense dead stars about the size of Earth in which all fusion has shut down, and are instead supported by the tight packing of their electrons, which, because they also act like waves as well as particles, refuse to get any closer together. However, if the mass of a white dwarf can be forced over the "Chandrasekhar limit" of 1.4 solar masses before it can release its layers by the nova explosion, the electron support becomes overwhelmed, and the white dwarf collapses, burns (in the nuclear sense) out of control, and annihilates itself in a powerful supernova. T CrB's white dwarf is close to the limit, so keep your eye out.

Less exotic perhaps, but just as intriguing, Corona Borealis is host to a triple-solar-star, Sigma, which consists of a pair of very close sunlike stars around which orbits a third near-solar clone. Corona Australis counters with fifth-magnitude Epsilon CrA, one of the two brightest "W Ursae Majoris" stars in the sky, an eclipsing binary in which two stars whirl around each other in only half a day so close together that they are actually in contact.

More exotic perhaps, the Northern Crown hosts one of the great nearby (at least by the standards of the Universe) clusters of galaxies, the "Corona Borealis Cluster," also known as Abell 2065. At a distance of a billion light years, A2065 recedes from us at a pace of 22,000 kilometers per second, its brightest member but 17th magnitude. Though large, 10 million light years across, it pales beside an even bigger structure 10 times that size, the Corona Borealis supercluster (of which A2065 is the main member of about 10 related systems) in which thousands of galaxies sprawl across the western curve of the constellation's semi-circle. Consistent with its status, A2065 was once actually two clusters that are in the final stages of merging, and (along with other galaxy clusters) is surrounded by intensely hot X-ray- emitting gas. Galaxy mergers are the most energetic of cosmic events, releasing a power that approaches a trillion supernovae.

The distant Universe is absent from the fainter Southern Crown. Because CrA is just off the main plane of the Milky Way, near the heart of the our Galaxy in Sagittarius, there is so much obscuring interstellar dust in the line of sight that other galaxies cannot be seen. It is instead the dust itself that is the star of the CrA show. The dust in interstellar clouds blocks starlight, turns the clouds cold, and promotes the manufacture of molecules, which are dominated by molecular hydrogen. At a distance of roughly 500 light years, the cigar-shaped Corona Australis molecular cloud spans five degrees (45 light years) across the northeastern portion of the constellation near the Sagittarius border. The whole interstellar mass tops 7000 Suns. Over half is concentrated at the western end roughly between Gamma and Epsilon in a much smaller volume that is associated with a complex set of pretty "reflection nebulae," in which dust grains reflect the light of neighboring or embedded stars. Within that is a "dense core" of around 50 solar masses.

The chilly temperatures within such clouds allow the molecule-saturated gas to contract, then collapse, in the creation of new stars, which can be identified by opposing flows coming out of circumstellar disks. Several such structures are found near the dense core, making Corona Australis a prime region to observe how stars come to be, from low mass bodies to relatively high mass ones that will someday heat the surrounding clouds to create bright diffuse nebulae for future astronomers to admire. While other constellations may be brighter and better known, the two Crowns together thus regally reveal the breadth of modern astronomy, from stellar youth to death, from nearby stars to distant galaxies that demonstrate the power and vastness of the Universe.
Copyright © James B. Kaler, all rights reserved. These contents are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes. First published in the August/December 2005 Newsletter of the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.