TAU-1 ERI (Tau-1 Eridani = 1 Eridani). We can extend the Greek-letter naming system by adding
sets of numbers that are commonly presented as superscripts.
Usually it's for two close stars, for example Alpha-1 and Alpha-2
Canum Venaticorum, the numbers going from west to east (Alpha-2
also known by its proper name, Cor
Caroli). Here and there are strings of them, the best known
Pi-1 through Pi-6 Orionis. Tau-1 Eridani, marking the eastern bend
of the River Eridanus, is first of the
co-champions, which are matched only by "Phi's" in northeastern Auriga. (The star is practically on the
border with Cetus, and so far east that
it is also, confusingly, Flamsteed's number 1, which has
nothing to do with the Greek additive.) The string of fourth and
fifth magnitude stars then goes from Tau-1 all the way through Tau-
9 at the eastern end, not far beyond which the River starts to
plunge to the south on its way to Achernar. Except for the names, the stars
in truth have nothing to do with one another, and as such represent
something of a random set that ranges from a blue class B6 dwarf
(Tau-8), through a red M3.5 giant (Tau-
4), to a much more solar class F (F6) dwarf, our Tau-1, which
at faint fourth magnitude (4.47) ranks fifth in brightness within
the string after the "luminary" (such as it is), reddish Tau-4
(which shines only at the bright end of magnitude four). The only
reason Tau-1 is bright enough to make the set is because of its
closeness, just 46 light years (give or take 1). With a
temperature of 6365 Kelvin, it shines just 2.6 times brighter than
the Sun, the radius coming in at 1.3 solar.
A projected rotation velocity of 25 kilometers per second give it
a rotation period of under 2.7 days. Stellar structure theory then
gives the star a mass of just 1.25 times solar and an age of 1.8
billion years, which is fairly young compared with its 4.9 billion
year hydrogen-fusing dwarf lifetime. Not one star, but two, Tau-1
Eri has a close companion about which nothing is known except for
a spectroscopically-determined
orbital period of 958 days. Assuming that it is low mass,
Kepler's Laws show that Tau-1 B orbits Tau-1 proper at a
distance of two Astronomical Units. If Tau-1 B contributes any
light to the system, Tau-1 A has an even lower luminosity and mass
than given above. Tau-1 Eri also sports a dust ring that may be
akin to our own Kuiper Belt (a ring of debris outside the orbit of
Neptune) that extends from 50 to
100 AU outward and might indicate a planetary
system of some odd sort with two inner suns, though none is
actually detected.
Written by Jim Kaler 12/24/10. Return to STARS.