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ALBALDAH (Pi Sagitarii). Among the brighter stars ofSagittarius is one that does not belong to the mainfigure, but floats above it to the north of theecliptic, helpingthe celestial Archer to span the solar path. Albaldah, to whichBayer assigned the Greek letter "Pi," is, however, the thirdmagnitude (2.89) luminary of the small asterism "the Teaspoon,"which goes along with Sagittarius's lower-down and larger famed"Teapot." Though the proper name is not standard in the recognizedliterature, it is still of ancient lineage. Arabian star loreestablished 28 "manzils," or "daily stations," or "mansions" forthe Moon in its path along the ecliptic. Number 21, "Albaldah" or"Al Baldah," gave its name to the star we now call "Pi Sagittarii,"and refers to "the City," in fact taken commonly as referring toMakkah, or Mecca. Albaldah is an impressive class F (F2) "brightgiant" 440 light years away that shines with the light of almostexactly 1000Suns from a white 6500 Kelvinsurface. These characteristics conspire to reveal a mass of fivetimes that of the Sun and an age of 95 million years from the timeit began life as a class B4 hydrogen-fusingdwarf. Pi Sagittariihas now shut down its core hydrogen fusion and is in transitionwith a dead helium core into becoming a classic bright helium-fusing "redgiant." Most interestingly, on a graph of stellarluminosity against temperature, the star is on the "blue edge" ofthe "Cepheid instability strip," the zone where stars likeDelta Cephei,EtaAquilae, andZeta Geminorum(Mekbuda) lose their sense of equilibrium and pulsate andchange their brightnesses like well-oiled clocks. Albaldah shouldbecome one of their number in about 1.5 million years. Two veryclosecompanionsaccompany the star, one at a separation of 0.09seconds of arc, the other a 6th magnitude star at 0.4 seconds. Nothing else is known about them. From its brightness, the outerone, at least 54 Astronomical Units away from Pi proper, is a classB9 star. The inner one, at least 13 AU out, most likely is aswell, but who knows. Given the close-in clumping of the stars, aswell as the lack of three-dimensional positioning, orbital periodsare near-impossible to guess, but would be at least 15 years forthe inner, and over a century for the outer.
Update: The second Hipparcos reduction gives a distance of 510 light years (16 percent higher) with an uncertainty of 34 light years. With an improved temperature of 6670 Kelvin, the luminosity rises to 1340 times that of the Sun, the radius is 28 times solar, and the mass creeps up to perhaps 5.2 Suns.Written byJim Kaler, 10/11/02; update added 8/15/14. Return toSTARS.
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