TAU ORI (Tau Orionis). Almost hidden by the glare of supergiant Rigel, the blue-white luminary of Orion, the Hunter, fourth magnitude (3.60,
almost third) Tau Orionis bears a faint relationship with Eridanus, the River, whose source is marked
by the star Cursa, Beta Eridani. Lying
just northwest of Rigel, Cursa is known as Orion's Footstool. In an expanded version, the Footstool is
also made of a four-star box that consists of Lambda and Psi
Eridani, and most likely Tau Ori. (In his great book of star
names, Allen leaves out the Greek letter!) As a class B (B5) giant
much less luminous than supergiant Rigel, Tau Ori is also closer,
though lying a still-healthy 555 light years away. While not
listed as a variable star, the Hipparcos satellite measured it as
just over a tenth of a magnitude brighter (3.47), so it may well
be. And though no match for Rigel, the star is still quite
luminous, shining with the power (including a lot of ultraviolet
light from its 14,100 Kelvin surface) of 3100 Suns. The radius is then deduced to be 9.4
times that of the Sun, which leads to a rotation period (from a
projected equatorial velocity of 43 kilometers per second) of under
11 days. Temperature and luminosity then tell of a six solar mass
star that has just ended its core hydrogen-fusing life and now
beginning its transition to brighter red-gianthood. Now around 63
million years old, it started life as a class B3 dwarf only a third
as luminous as it is today. Well under the limit for supernova explosion, the star
will eventually shed its outer layers to become a relatively heavy
white dwarf of around
0.95 solar masses, not unlike the companion to Sirius. The multiplicity appears at first
complex. Observations nearly a century old suggest a variable
velocity and a close-in orbiting spectroscopic companion, but it has
never been confirmed. The star is also listed with three visual companions. Relative
motion between Tau Ori A and 11th magnitude Tau Ori B (34 seconds
of arc away) shows the fainter star just to lie just in the line of
sight. Tau C, an apparent companion to Tau B, must be coincidental
as well. The leaves 11th magnitude Tau Ori D, which at a
separation of 36 seconds of arc has tracked Tau proper for well
over a century and is probably a real companion. If so, it must be
a solar type star. Lying at least 6100 Astronomical Units away
from Tau A, it would take at least 180,000 years to make a full
circuit. From the bright star, Tau D would shine with the light of
more than 5 times that of Venus, while from Tau D, Tau A would
exceed the brightness of 15 of our full Moons.
Written by Jim Kaler 01/23/09. Return to STARS.