POLIS (Mu Sagittarii). Sagittarius's bow
is mainly outlined by three stars, Kaus
Borealis to the north, Kaus Media at
the center, Kaus Australis to the south.
But it is a classic bent bow, the upper part curling outward and
ending at Polis, which is better known as Mu Sagittarii. The
Coptic name, which means "a foal," is meant to describe the stars
of the bow, but has instead devolved to this most northerly of the
bow's stars. A mighty star, Mu Sagittarii is still not very well
understood. Shining at mid-fourth magnitude (3.83), it is a very
close eclipsing double that has three, possibly four, visual
companions. Traditionally classed as a bright-side class B (B8)
supergiant, it is now listed as a more ordinary giant. But it is
so far away that its distance cannot be directly measured, at least
3000 light years, so it must be vastly brighter than a giant would
be. The best distance estimate comes from the star's membership in
the Sagittarius OB 1 association (a loose organization of similar
stars), which places it around 3600 light years away, rendering it
a true supergiant, which is what we shall adopt. At that distance
it shines with the light of 180,000 Suns,
after correction from some ultraviolet light from its 11,100 Kelvin
surface and further correction of 0.17 magnitudes (17%) for dimming
by interstellar dust. The luminosity and temperature conspire to
give a radius 115 times that of the Sun and mass 23 times solar.
As a blue supergiant with a dead helium core, the star (only 8
million years old) is making its transition to becoming a much
bigger red supergiant, whereupon it will fuse its helium to carbon,
and then on to iron, whence it will explode as a brilliant
supernova. Every 180 days, the star suffers a small dip, as a
smaller, much fainter and close companion (most likely a B1.5
eight-solar-mass dwarf) cuts in front of massive Mu itself. The
two orbit at a distance about twice that of Earth from Sun. As are
all supergiants, Polis is losing mass via a strong wind, through
which it loses about a millionth of a solar mass a year at a wind
speed of 600 kilometers per second. Far beyond the close double
are Polis B (magnitude 8.04 at 17 seconds of arc, at least 29,000
Astronomical Units away), C (10.99, 26 seconds, 44,000 AU), D
(9.5?, 47 seconds, 80,000 AU), and E (9.25, 51 seconds, 87,000 AU).
All are class B dwarfs. Polis D is no longer listed with the
group, and may just be line of sight. The Polis group is a
"Trapezium" system, in which the stars more mill about than orbit
a central mass. All such are doomed to extinction, as first one
star, then another is kicked out until only one or two real
orbiters are left. Thanks to Jose Rodriguez, who suggested this
star.