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.