EPS ARI (Epsilon Arietis). Double stars run the gamut of separations and brightness differences, from components that nearly touch each other to "fragile" cohabitations such as the tie between Proxima Centauri and the far brighter and more massive binary (Alpha Centauri) that it orbits; from nearly equally bright companions to those like Sirius, where one is ten thousand or more times brighter than the other. Epsilon Arietis, a fifth magnitude (4.60) binary, which lies well to the east of the classic figure of Aries (the Ram), is of the visually-close-nearly-equal variety. Of the same class, both white type A (A3) hydrogen-fusing dwarfs, fifth magnitude (5.17) Epsilon A (the eastern of the two) is just 0.4 magnitudes brighter than Epsilon B (5.57). In the nineteenth century, Smythe and Chambers called them "pale yellow and whitish," the coloration a contrast effect. F.G.W. Struve, who discovered the duplicity in 1827 when they were a mere 0.5 seconds of arc apart, referred to them as the "closest of all neighbors." A current separation of just 1.4 seconds (the two in slow orbit) still makes any kind of measure of the individuals quite difficult, the "A" and "B" even closer than the individual binaries of Epsilon Lyrae. And as a result, very little is known about this lovely double. There being no actual temperature measures, 9100 Kelvin is adopted from the class. That and a distance of 342 light years (give or take 24) give respective luminosities for Eps A and B of 80 and 55 Suns, with radii of 3.6 and 3.0 solar. A measure of projected equatorial rotation speeds of 60 kilometers per second for both (or for one or the other, it's hard to say) give rotation periods under 3.0 and 2.5 days, fast enough with unknown axial tilts to keep their atmospheres sufficiently stirred to prevent peculiar abundances as a result of gravitational settling and radiative lofting of various chemical elements. Theory gives respective masses of 2.7 and 2.5 Suns and an age of 420 million years. Since the hydrogen-fusing lifetime of more- massive Epsilon A is only 500 mega-years, it does not have long to go before turning into a subgiant with a dead helium core and then a true red giant, when it will overwhelm Epsilon B.

The relative movement of the two gives an orbital period of 1216 years at an average separation of 228 Astronomical Units, an orbital tilt of 84 degrees to the plane of the sky (which may well not be shared by the stellar rotations), and an eccentricity that runs the stars between 300 and 156 AU apart. Kepler's laws then yield a mass-sum of 8.0 times that of the Sun, too high, revealing quite-understandable orbital errors. Adopting the evolutionary sum of 5.2 Suns and assuming the orbital period to be correct (which it may well not be), the orbital size shrinks to 196 AU. Out at a great distance of at least 15,000 AU lies 13th magnitude Epsilon Ari C, which for a century has kept close pace with its far brighter mates and may well belong to the system. C's absolute brightness would make it a low-mass K7 dwarf with an orbital period of at least 800,000 years, and we are back to a system rather like Alpha and Proxima Centauri. At the nominal values, from Epsilon C, the A-B team would appear as two Venus's separated at most by around 4 minutes of arc. Most likely, though, nobody is there to watch. (Thanks to Tim Benedictis for suggesting this star and to David Bright for a Latin translation.)
Written by Jim Kaler 3/18/11. Return to STARS.