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
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-
giant." Most interestingly, on a graph of stellar
luminosity against temperature, the star is on the "blue edge" of
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
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.
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 by Jim Kaler, 10/11/02; update added 8/15/14. Return to STARS.