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.
Written by Jim Kaler. Return to STARS.