TAU-5 ERI (Tau-5 Eridani). The string of fourth and fifth magnitude stars that make the easterly flow of the River Eridanus runs from Tau-1 Eri at the eastern end to Tau-9 at the western, the number of numbers equalled only by equally faint Phi-1 to Phi-9 Aurigae, though Tau-1 through Tau-8 Serpentis in western Serpens (Serpens Caput) come close. Physically, Eridanus's "Tau's" range from mid class B (B6) Tau-8 to the near- solar class F dwarf Tau-1 Eri and the red giant Tau-4. Among the more unusual of the set is fourth magnitude (4.27) Tau-5. Ranking fourth in brightness within the gang, it consists of a pair of seemingly identical blue-white class B (B8) dwarfs in a very tight 6.224-day orbit, the duplicity detected only spectroscopically. If they are identical (and there is one dissident voice), each would be seen seperately as mid-fifth magnitude (5.02), which we hereafter adopt. With no measure of actual temperature, we can only assume one of 12,100 Kelvin from the spectral class, which in turn gives the amount of invisible ultraviolet light radiated by the fairly hot surfaces. Each star thus sends the total light of 120 Suns into space, from which we calculate radii of 2.5 times solar. Projected equatorial rotation speeds of around 45 kilometers per second show the average rotation periods to be under 2.8 days. Masses (from stellar structure theory) must then be around 3.2 solar, theory also suggesting a mutual age of under 100 million years, well short of the 300-million-year hydrogen-fusing lifetime. The stars may be even younger. Application of Kepler's Laws show that the average separation between them is just 0.12 Astronomical Units, less than a third Mercury's distance from the Sun. Yet there is a significant orbital eccentricity of 0.2, which takes them as far apart as 0.15 AU and as close as 0.10 AU. Because of mutual gravitational forces, orbits of close binaries tend to circularize with time, and these two have not yet achieved that state. Moreover, the rotational and orbital periods are at least a factor of two from being synchronized (though there is considerable disagreement in rotation velocities). Yet, for all the seeming understanding of the system, not all is well. One dissident source suggests that the pair is made of B0 and B9 dwarfs. Class B stars, however, have a huge range in luminosity, even visual luminosity. To the eye, Tau-5 A would then be at least 60 times brighter than Tau-5 B, rendering the fainter star nearly, if not totally, invisible. Most likely, they are in fact identical. Their fates are uncertain. As they age and (when their core hydrogen fuel supplies give out) swell into giants, they will encroach upon one another, interact, and both lose and exchange mass, which will grossly alter the courses of their evolution, the result not really calculable. All we can do is wait a long, long time to find out.
Written by Jim Kaler 1/14/11. Return to STARS.