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.