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.