UPS ORI (Upsilon Orionis). Few constellations are as full of massive stars as Orion, the result of recent and sequential star formation that is playing out before and within the black curtain of the Orion Molecular Cloud. Here we find Theta-1 Ori C (the exciting star of the Orion Nebula), the three stars of the Belt, Rigel, Betelgeuse, Lambda and Sigma Ori, and many more. If we probe deeply enough, we reach lonely fifth magnitude (at 4.62, just over the line from fourth) Upsilon Orionis, which lies south and a bit west of the Sword. (Don't confuse it with U Orionis, a long period variable far to the north.) The proper name "Tabit" (from Arabic mysteriously meaning "the Endurer") has been ascribed to it, but it's also been applied to brighter Pi-3 Orionis, where we will leave it, and stick with the Greek letter, fifth from last in the alphabet. The star is a whopper, a luminous class B0 dwarf (right at the edge of class O) at a most likely distance of 2960 light years. It's so far away, however, that parallax becomes unreliable, the formal uncertainties placing it as far as 3670 light years or as close as 2350. Upsilon may be part of the Orion OB1c association of hot massive stars, those in the region of Orion's Sword, but even at its statistically closest, it's notably farther than the Sword association's mean distance of 1650 light years. Given how far away it is, it's a bit surprising that Upsilon is dimmed just 0.13 magnitudes by interstellar dust. From its magnitude, the distance, and a temperature of 32,440 Kelvin needed to account for a lot of invisible ultraviolet radiation, we find a huge luminosity of 173,000 times that of the Sun, double that of Betelgeuse and Rigel, though not reaching the level of Theta-1 C. (At its likely closest, the luminosity drops to 116,000 Suns, still pretty impressive.) Temperature and luminosity then conspire to give a radius of 13.2 times solar. The projected equatorial rotation speed of just 15 kilometers per second is low, probably because the rotation pole is more or less directed at us, as for such stars we would expect more than ten times that speed. Theory tells of a huge mass of 30 Suns, with distance uncertainties allowing a range of 25 to 35. The great luminosity forces a wind that blows billions of times the strength of the solar wind. Now about halfway through its 5.5 million year dwarf lifetime, there is little question that Upsilon Orionis is going to blow up someday as a grand supernova, leaving behind a neutron star or even a black hole. Unfortunately, there seems to be no companion to interact with the proceedings and perhaps to tell us more.

Written by Jim Kaler 3/14/14. Return to STARS.