THE ROYAL SKY
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.