Movatterモバイル変換

[0]ホーム

ALYA (Theta Serpentis). A uniqueconstellation,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 Serpent Bearer), the western portion is Serpens Caput (the Serpent's head), the eastern Serpens Cauda (the tail). TheGreek letter names are distributed between the two portions: Alpha through Epsilon are in Serpens Caput, while thelettering in Serpens Cauda does not start until Zeta. Serpens (not a terribly bright constellation) has only two named stars. The Alpha star (Unukalhai) is in Serpens Caput, while Alya (Theta Serpentis) is in Serpens Cauda. Oddly, none of the other stars, several of which are brighter than fourth magnitude (3.99) 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 an erroneous relation toAlioth inUrsa Major and inappropriately given to Theta Ser. (Alya in Arabic also means "high," "holy, a "lofty thing," the star name coming from a similar transliteration of a different word.) Continuing the duplicity, Alya is a marvelous double star worthy ofMizar in theBig Dipper. Even a small telescope shows nearly identical fifth magnitude Class A (A5) hydroben-fusingdwarf components separated by 22.4 seconds of arc, the western (Theta-1) just a bit brighter (4.59) than the eastern (Theta-2, 4.93, the two also respectively called Theta A and B).

The fine double star Alya (Theta Serpentis) shines just above center, its two white components 22 seconds of arc apart. Only a tiny amount of orbital motion has been recorded over the past 2.6 centuries. North is down. Slightly brighter Theta-1 (Alya A) is on the left, Theta-2 (Alya B)is at right. The spikes on the stars are artifacts caused by struts that support the telescope's secondary mirror. See thefull image. Thanks to Steve Lewis of Ft. Worth Texas.

From the nineteenth century, Smythe and Chambers write: "A neat double star in the Serpent's tail...A 41/2 pale yellow; B 5 golden yellow...a fair and easy object for a moderate telescope." Both are actually white with a mean temperature (for simplicity adopted for both) of 7520 Kelvin and are dimmed by roughly 0.11 magnitudes by theinterstellar dust of theMilky Way. With a distance of 162 light years (a straight average that implies an uncertainty of 7 light years), they shine with respective luminosities of 33 and 24 times solar, which in turn give diameters of 3.3 and 2.9 times that of theSun. Theory yields masses of 2.2 and 2.0 Suns. Both are nearing the ends of their hydrogen-fusing lifetimes, more massive "A" farther along. Most likely having been born under similar conditions, they are also rapid rotators. Theta-1 (Alya A) spins with an equatorial speed of at least 141 kilometers per second, while Theta-2 (Alya-B) rotates substantially faster, 217 kilometers per second (more than 100 times the solar value). Rotation periods must then be under 1.1 and 0.7 days, short enough to stir up the stellar gases and prevent separation of elements. There is no doubt about the stars' physical relation as they have changed their separation by a mere 0.3 seconds of arc over the past 260 years (the relative motion nowhere near enough to allow the calculation of actual orbital parameters). The two stars must be separated by at least 1100 Astronomical Units (28 times Pluto's average distance from the Sun), and must therefore orbit with a period of at least 18,000 years. If they are 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 11full Moons, allowing them to easily read their issues of the "Alya Gazette" at night. Seven minutes of arc away is seventh magnitude Alya C. From its motion, its apparent companionship is merely a line of sight coincidence. Alya is coming at us with a velocity of 52 kilometers per second but is also shifting to the north-northeast at 11 km/s. In about a million years it will pass us at a distance of some 30 light years. The pair might then be barely separable with the naked eye, each gloriously shining at first magnitude.

Written byJim Kaler 9/1/00; revised 10/29/15. Return toSTARS.

[8]ページ先頭