OMEGA ERI (Omega Eridiani). One would think all reasonably bright stars would be understood. Not so. Here's one with an enduring mystery, one that stares back at us from a prominent position just south of the celestial equator. Little attention is paid to Omega Eridani, the last-Greek-lettered star in Eridanus, the River. Just west of Cursa (Beta Eri), fourth magnitude (4.39) Omega is not usually included in the "dotted-line outline" of the constellation. Even its class is problematic. The Yale "Bright Star Catalogue" persistently lists it as having a composite class A-F giant-star spectrum (F4 and A6), as if two giants were in orbit around each other, which given that giant status is fairly fleeting, would be unusual. More reasonably the star is also classed as a single class A (A9) subgiant, implying that it has ceased core hydrogen fusion and that it is on its way to becoming an actual giant. Rotating very quickly (at least 180 kilometers per second at the equator), the spectrum lines from which the star is classified are broad and washed out, which is perhaps responsible for the classification anomaly. (The "Doppler effect" causes shifts in the wavelengths of light as a result of motion toward or away from the observer.) Assume a single star. Shining from a distance of 227 light years, with an estimated temperature of 7500 Kelvin (consistent with the lack of interest in the star, it has never been measured), Omega Eri has a respectable luminosity 65 times that of the Sun, which gives it a radius 5 times solar and a rotation period of less than 1.4 days. Omega then fits expectations of an A9 subgiant with a mass of 2.5 solar. So far so good. But Omega really IS a binary, or at least so it seems. Doppler shifts of the spectral absorptions tell of a companion with a period of 3057 days (8.4 years). Nothing is known of it except that the mass should be at least 3.5 times that of the Sun. Yet the "companion" is not visible. It is too massive for a dead white dwarf as such stars are limited to under 1.4 solar masses. Decades ago, the star was a candidate for having an orbiting neutron star or black hole! Yet there is no other evidence, including the expected X-ray emission that might come from stellar interaction. It could be that the washed out spectrum is just fooling us too and that the star is really single. And since no one has made much attempt to find out (Omega Eri subject to almost no current research), this modest star, one made easy to find by its proximity to Orion's Rigel and Cursa, remains an enigma.
Written by Jim Kaler. Return to STARS.