OMI TAU (Omicron Tauri = 1 Tauri). If Omi Tau, an otherwise rather ordinary fourth magnitude (but at 3.60 almost third) class G (G6) giant, has any great distinction, it's position. Acting as a celestial drum major, the star (really along with Xi Tau) leads the parade of Taurus's grand sights across the sky. This western-most of the Bull's naked-eye stars is tucked right against the border with Cetus (almost where Taurus, Aries, and Cetus come together), hence is also Flamsteed's number 1 (Xi number 2). Look just to the west of it to find the Sea Monster's round head. The quite well-known temperature of 5705 Kelvin (Omi Tau a popular destination for temperature determinations) tells of a small correction for infrared radiation. Factoring in a distance of 212 light years gives a luminosity of 154 times that of the Sun, a radius of 16 times solar, and a ponderous rotation of (at most) 135 days, actually rather fast for a giant. The theory of stellar structure and evolution then gives a mass between 3.2 and 3.4 times that of the Sun. We can't tell, however, whether the star is about to brighten with a dead helium core (at 3.4 solar masses) or has already fired up its helium to burn to carbon and oxygen (in which case 3.2 solar) and has settled into a relatively long equilibrium (though far shorter than its 280 million year hydrogen fusing life, begun when the Omi Tau was a youthful hot, blue, class B6 dwarf). The metal abundance is near normal, about 80 percent solar. Whatever its status, Omicron Tauri is accompanied through life by a small spectroscopic companion that orbits its sibling with a period of 1654 days, or 4.53 years. Nothing is known about it. Guessing a mass of one-half solar, the companion orbits at a mean separation of 4.3 Astronomical Units, a modest eccentricity of 0.27 taking it between 5.4 and 3.1 AU (in our Solar System scale, roughly from Jupiter to the middle of the asteroid belt). The star is large enough that from the companion it would still appear two degrees across (as opposed to our one- half-degree Sun).
Written by Jim Kaler 3/20/09. Return to STARS.