ALYA (Theta Serpentis). A unique constellation, Serpens (the Serpent) is the only one that is divided into two parts, neither of which touch the other. Entwined around, and separated by, Ophiuchus, the western portion is Serpens Caput (the Serpent's head), the eastern Serpens Cauda (the tail). The Greek letter names are distributed between the two portions: Alpha through Epsilon are in Serpens Caput, while the lettering in Serpens Cauda does not start until Zeta. Along with two portions, Serpens (not a terribly bright constellation) has only two named stars. The Alpha star (Unukalhai) is in Serpens Caput, while Alya (the Theta star) is in Serpens Cauda. Oddly, none of the other stars, 6 of which are brighter than Alya, are named. Extending the oddness, the name Alya has nothing to do with a snake, but comes from an ancient Arabic word for the tail of a sheep, the term descended from erroneous relation to Alioth (in Ursa Major) and inappropriately given to the star. ("Alya" in Arabic also means "high," "holy," a "lofty thing." The star name comes from a similar transliteration of a different word.) Continuing the duplicity, Alya is a marvelous double star worthy of Mizar in the Big Dipper. Even a small telescope shows nearly identical fifth magnitude components, the western (Theta-1) just a bit brighter (4.62) than the eastern (Theta-2, 4.98). The same color (white) and temperature (about 8200 Kelvin), they have identical spectral classes, both class A (A5) ordinary hydrogen- fusing dwarfs like the Sun. At a distance of 130 light years, they shine with respective luminosities of 18 and 13 times solar, have diameters each about twice that of the Sun, and masses also around double solar. Most likely having been born under similar conditions, both are also rapid rotators. Theta-1 (which, since it is the brighter of the two could also be called Alya-A) spins with an equatorial speed of at least 143 kilometers per second, while Theta-2 (Alya-B) rotates a bit faster, 196 kilometers per second, almost 100 times greater than does the Sun. If the rotation axes are perpendicular to the line of sight, then their rotation periods are a mere 17 and 12 hours respectively. The two stars are separated by at least 900 Astronomical Units (23 times Pluto's average distance from the Sun), and must therefore orbit with a period of at least 14,000 years. It is little wonder that no orbital movement has actually been seen. If they are actually at their minimum separation, residents of a hypothetical "earth" orbiting either one would see the other shine in their skies with the brightness of some 16 full Moons, allowing them to easily read their issues of the "Alya Gazette" at night.
Written by Jim Kaler 9/1/00, revised 7/25/08. Return to STARS.