ALBALDAH (Pi Sagitarii). Among the brighter stars of Sagittarius is one that does not belong to the main figure, but floats above it to the north of the ecliptic, helping the celestial Archer to span the solar path. Albaldah, to which Bayer assigned the Greek letter "Pi," is, however, the third magnitude (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 recognized literature, it is still of ancient lineage. Arabian star lore established 28 "manzils," or "daily stations," or "mansions" for the 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 to Makkah, or Mecca. Albaldah is an impressive class F (F2) "bright giant" 440 light years away that shines with the light of almost exactly 1000 Suns from a white 6500 Kelvin surface. These characteristics conspire to reveal a mass of five times that of the Sun and an age of 95 million years from the time it began life as a class B4 hydrogen-fusing dwarf. Pi Sagittarii has now shut down its core hydrogen fusion and is in transition with a dead helium core into becoming a classic bright helium- fusing "red giant." Most interestingly, on a graph of stellar luminosity against temperature, the star is on the "blue edge" of the "Cepheid instability strip," the zone where stars like Delta Cephei, Eta Aquilae, and Zeta Geminorum (Mekbuda) lose their sense of equilibrium and pulsate and change their brightnesses like well-oiled clocks. Albaldah should become one of their number in about 1.5 million years. Two very close companions accompany the star, one at a separation of 0.09 seconds of arc, the other a 6th magnitude star at 0.4 seconds. Nothing else is known about them. From its brightness, the outer one, at least 54 Astronomical Units away from Pi proper, is a class B9 star. The inner one, at least 13 AU out, most likely is as well, but who knows. Given the close-in clumping of the stars, as well as the lack of three-dimensional positioning, orbital periods are near-impossible to guess, but would be at least 15 years for the inner, and over a century for the outer.
Written by Jim Kaler. Return to STARS.