TAU-4 ERI (Tau-4 Eridani). The 24 Greek
letters used originally by Bayer to name stars within a given
constellation are commonly extended by
using numbers (usually expressed as superscripts). The sky abounds
with them, examples being Epsilon-1 and
Epsilon-2 Lyrae, Alpha-1 and Alpha-2
Capricorni. Usually they run from west to east, the outstanding
exception being the string of Pi-1 to Pi-6
Orionis, which runs north to south. The champion among the
numbered sets, though, is Tau-1 through Tau-9 Eridani, which
together mark a west-to-east flow in the River Eridanus. Among these fourth and fifth magnitude
stars, the brightest, in the middle of this part of the stream, is
fourth magnitude (3.69, almost third) Tau-4 Eridani. A red, class
M3.5 giant (that Smythe and
Chambers call "light orange"), it is not only by far the coolest of
the lot, but is also in a most interesting state of evolution.
(The faintest is fifth magnitude Tau-7, while among the more
interesting is Tau-5, a close spectroscopic binary made of twin B8
dwarfs.) At all different distances, the stars have nothing to do
with one another except proximity and mythology. At a distance of
258 light years, Tau-4 Eri, largely as a result of its gianthood,
is an irregular variable that wanders between magnitudes 3.59 and
3.72. With a temperature estimated at 3575 Kelvin, the star shines
with the light of 1110 Suns, most of it in
the infrared part of the spectrum. Temperature and luminosity then
conspire to give a radius 87 times that of the Sun, 0.40
Astronomical Units, just a bit bigger than the orbit of Mercury.
With a mass of 1.5 times that of the Sun and an age of 2.7 billion
years, around half a million years ago the star gave up core
hydrogen fusion. It now appears to be near the end of a
brightening phase, which will be terminated when its current dead
helium core (surrounded by a shell of fusing hydrogen) fires up to
fuse to carbon and oxygen. The star will then dim some as it
settles down to become one of the many orange class K giants, such
as Arcturus and Aldebaran, that dot the sky. Catalogues
list as many as five companions. All but Tau-4 B,
however, are (or are probably) line of sight coincidences. The "B"
component, however, seems to be real. At a separation of 5.9
seconds of arc, this tenth magnitude (9.5) class K0 dwarf is at
least 470 AU from much brighter Tau-4 A, and must then take at
least 6500 years to make a full orbit. From the companion, Tau-4
A would appear as a tiny reddish disk no bigger than six minutes of
arc across that shines with the visual light of 350 full Moons (the infrared adding a lot
of extra heat!)
Written by Jim Kaler 2/20/09. Return to STARS.