TAU SGR (Tau Sagittarii). Among the night sky's prettiest figures is the small five-star "Little Milk Dipper" in Sagittarius. The small asterism, with its handle sticking in the Milky Way, makes the Archer immediately recognizable. The brightest three of the quintet carry traditional and rather well-known proper names (in order: Nunki, Ascella, and Kaus Borealis, respectively Sigma, Zeta, and Lambda) that are lacking in the fainter two, Phi Sagittarii and (at the brightness end) Tau Sgr. Though the faintest of them, Tau Sgr still shines nicely at third magnitude (3.32) from a distance of 120 light years. A rather common class K (K1, though sometimes listed as K1.5) orange giant with a temperature of 4440 Kelvin, the star radiates at a rate of 92 times that of Sun, from which we derive a radius of 16 times solar. Combination of luminosity and temperature (plus a lot of theory) clearly shows that Tau Sgr is in state of fusing the helium in its core into carbon and oxygen and that it is a stable "clump giant," the term coming from the large number of stars in a similar situation, Arcturus and then Aldebaran leading the lot. At this stage in stellar life, mass makes little difference in brightness, making the mass difficult to gauge. The best estimate falls between 1.5 and 2 times that of the Sun, giving an age between 1.3 and 2.8 billion years. There is some slight indication that the starlight is absorbed by a bit of interstellar dust in the Milky Way that floods through Sagittarius, which would raise the luminosity upward by no more than 20 to 30 percent and the mass up to perhaps 2.5 solar. Like most mature giants, Tau Sgr is rotating slowly. Nevertheless, the spin speed has been measured at a leisurely 3 kilometers per second (the minimum, since the axial tilt is not known), which gives a rotation period of up to 270 days, over 10 times the 25-day rotation period of the Sun. The star is sometimes listed as a spectroscopic double (the companion inferred from the behavior of the spectrum), and is on a list of suspected doubles observed by the Hipparcos parallax satellite, though in spite of careful ground-based observations no companion has been sighted. Tau Sgr's most significant characteristic is a high velocity of 64 kilometers per second relative to the Sun, more than four times the local average, suggesting that the star is a visitor from a different part of the Galaxy. Consistently, it has a lowered metal content, the iron-to-hydrogen ratio measured at 70 percent that of the Sun.
Written by Jim Kaler. Return to STARS.