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.