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.