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The stellar ties that bind....

With only one constellation each to now call home, the stars Alpheratz and Elnath once carried dual citizenship.

Every month, our gatefold celestial chart cinches the sky in a star-sequined corset of astronomical coordinates. With hoops that girdle the night and ribs that radiate from the north celestial pole, this halter keeps everything in place with a web of reference circles tied to the Earth's spin and orbital motion. The foundation of those stays and braces is the blue celestial equator. It splits the night into two hemispheres, and on our all-sky map it's a belt that binds the sky to the Earth at the east and west cardinal points on the horizon.

Each star is sewn to the sky with the help of the celestial equator. A pair of angles measured with respect to this seam specifies an object's position. One of them, declination, is the angle above (north) or below (south) the celestial equator. The second, right ascension, is the eastward equatorial span from the vernal equinox, one of the celestial equator's two intersections with the ecliptic. The Sun, Moon, and planets travel along the ecliptic, which is the green trim on the evening wear modeled by our Sky Map of the Month. The vernal equinox is in the constellation Pisces, the Fishes, and you can find it on the west flank of January's map. The great circle that connects the vernal equinox with the north celestial pole - and more-or-less hooks it to Polaris - is called the equinoctial colure. Although this strand is invisible, two conspicuous stars, Caph (Beta Cassiopeiae) and Alpheratz (Alpha Andromedae), are close enough to it to reveal the path the colure threads through the constellations.

Alpheratz marks the head of Andromeda, the rock-shackled princess of Ethiopia who was offered in sacrifice to appease a coast-ravaging sea monster. Traditionally depicted in cold irons bound, her constellation was usually known to the ancients only as Andromeda. Aratus, whose Phaenomena documents Greek sky lore for the third century B.C., calls her by name but reminds us her earlier confinement is still overhead on public display: "Yet even there she is racked with arms stretched far apart, and even in Heaven bonds are her portion." Centuries later, her fetters and her distress prompted Richard Hinckley Allen, in Star-Names and Their Meanings (1899), to call her the Woman Chained. Similar titles are, however, at least as old as late medieval Arabic translations of Ptolemy's Almagest, in which Alpheratz is named the Head of the Woman in Chains.

It's one thing to chain a princess to the rocks in deference to a leviathan, but Andromeda's bondage is also enforced by a flying horse. She is tethered to Pegasus by Alpheratz, a star both constellations share on the northeast corner of the Great Square of the Winged Horse. This star is now officially assigned to Andromeda, but in antiquity Alpheratz was not the property of a single owner. Aratus granted joint custody: "One common star gleams on the Horse's navel and the crown of her head" The unknown anthologist of the Catasterismi, the oldest surviving collection of Greek constellation myths (late first century B.C.), identified Alpheratz as Andromeda's head in his entry for her, and later, in his chapter on Pegasus, he assigned it to the edge of the Horse's belly. The Latin poet Hyginus echoed the concept of shared tenancy in the second century A.D. The ninth of Ten Books on Architecture composed by the Roman engineer Vitruvius in the first century B.C. is an astronomical digest, and its dual allocation of Alpheratz is explicit: "A very bright star terminates both the belly of the Horse and the head of Andromeda.' In the Almagest (second century A.D.), Ptolemy listed it only with the stars of Pegasus but noted it also belonged to the head of Andromeda. Alpheratz, then, has long curbed the independence of Pegasus and the Princess with one set of reins.

Constellations don't usually have to share stars, but Pegasus and Andromeda aren't the only pair of figures tied together by a stellar bond. A couple of stars in Ophiuchus, the Serpent Holder, and Serpens, the Serpent, appear to be performing double duty where the snake handler grips the coils. Modern usage, however, apportions these stars to either the snake or its charmer. The Catasterismi and the Poeticon astronomicon of Hyginus, on the other hand, sidestepped charges of stellar discrimination by handling the serpent handler and his viper as a single figure of stars.

Even if the points of ophidian contact with Ophiuchus were regarded as community property, the arrangement doesn't seem particularly awkward. He is, after all, juggling a snake. Taurus, the Bull, and Auriga, the Charioteer, however, can't really justify time-sharing the star Elnath.

While Elnath ornaments the tip of the north horn of the Bull, it also permanently pierces Auriga's right heel. That kind of distraction makes it hard to keep your mind On the road. Aratus and Vitruvius explained the star is common to both constellations, and Ptolemy highlighted the same tight connection by listing it twice.

The ancient Greeks conferred dual citizenship on Alpheratz and Elnath, but their reason for buttoning independent constellations together is no longer dear. We may be seeing what happens when independent traditions for mapping constellations merge. In any case, there are actually closer relationships and tighter ties that bind stars together than the configurational accidents sustained by Elnath and Alpheratz. Alpheratz, for example, is actually a spectroscopic binary, two stars sharing the intimacies of gravity too closely to be distinguished as independent partners in a telescope eyepiece. Although they cannot be directly resolved, their Doppler-shifted partnership is revealed through spectral analysis.

Many stars - perhaps half of them - are pairs, or even trios. Gravity binds them to each other in mutual orbit, and the distance between these components has a lot to do with how they behave. When a binary is separated by a distance as large as the diameter of our solar system or more, its members respond to their mutual gravitational attraction but don't interfere with each other's development. When the distance between them is much less, however, the relationship is more complex and less healthy. Closely bound by gravity, the stars in such binaries may exchange material or even share a common envelope and in that sense are in contact. Explosively close, these pairs affect each other's character.

A crucial dimension of these pairs is the distance at which the gravitational influence of one star is matched by the other. The region defined by this equilibrium boundary is the so-called Roche lobe, named after the 19th-century French mathematician Edouard Roche, in which material is retained by the star within it. If the star's gas somehow reaches this boundary, it can flow to the companion. We now know that cataclysmic variables - stars that rapidly and temporarily explode into far greater brilliance - are contact binaries in which one member is overflowing its Roche lobe. The other component is already a white dwarf, an evolved and compact object. With roughly the mass of the Sun packed into a volume the size of the Earth, a white dwarf cannot long endure the burden of material added by its overflowing partner. Many different transactions between them are possible, but these options all culminate in an explosive dispersal of material and energy from the territory of the white dwarf. Novae and related events are explained by these systems.

One type of supernova - the catastrophic destruction of a star - can also occur when material from a companion is loaded too rapidly on a white dwarf in a highly evolved pair. Nothing remains after the explosion but an expanding bubble of wisps and tatters.

The collusion in dose binaries inevitably leads to erratic behavior. Whether they are bound for astrophysical glory or are on stellar evolution's road to ruin, gravitational fidelity enchains white dwarfs, neutron stars, and black holes in exotic ribbons and disks of flowing gas. These ties bind them to a sideshow lifestyle unless a runaway explosion ends the alliance.

E. C. KRUPP is attached to Griffith Observatory in Los Angeles.
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Title Annotation:reassignation of Alpheratz and Elnath to a single constellation
Author:Krupp, E.C.
Publication:Sky & Telescope
Date:Jan 1, 1999
Words:1354
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