THETA UMA (Theta Ursae Majoris). Midway up the front leg of Ursa Major, the Great Bear, to the north of
one of the Arabs' three "leaps of the gazelle" (a trio of star
pairs) lies mid-third magnitude (3.20) Theta Ursae Majoris. It has
no proper name of its own, but shares one with five other stars
that the Arabs called (from Allen) Thufr al Ghizlan, the Throne of
the Mourners (the Dipper the Arabs' "funeral bier"). This triple
star system is dominated by a coolish (6370 Kelvin), somewhat-solar
class F (F6) subgiant. Indeed, Theta UMa is one of the brighter
stars in the sky that even closely replicates our own Sun, something to appreciate when viewing the
entirety of the Great Bear. Only 44 light years away, the main
star shines with the light of 7.6 Suns, its radius about double
solar. The luminosity and temperature tell of a star with a mass
of between 1.5 and 1.6 solar that is near the end of its three-
billion-year hydrogen fusing lifetime. If the core fuel is not
used up already, it soon will be, consistent with the "subgiant"
status determined from the spectrum. Theta UMa is also a bit low
in the metals department, its iron abundance only 60 percent that
of the Sun. Set at an angular separation of 4.1 seconds of arc is
a very faint 14th magnitude (13.8) companion. While no orbital
motion has been perceived, the two move through space together and
are most likely a real, though odd, couple, as from its brightness,
the companion is a low-mass (about 15 percent solar) class M6 red
dwarf about which nothing else is known. As in all such
observations, the angular separation gives a minimum to the
physical separation because of likely foreshortening. The stars
are therefore at least 55 Astronomical Units apart. Statistical
adjustment for the foreshortening gives a most likely separation of
94 AU, which leads to an orbital period of 700 years (explaining
the lack of clear orbital motion). The spectrum of the Theta UMa
proper also indicates a close-in companion with a period of only
371 days, suggesting a separation somewhat greater than an
Astronomical Unit, nothing known about this star either. From the
farther-out 13th magnitude companion, the bright star would shine
with the light of 1000 full Moons with the close-in companion at
most a half a degree away, while from Theta proper, the outlier
would shine redly with luminosity of only 10 times that of Venus.