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.