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.