BEID (Omicron-1 Eridani). In the middle of the first southerly
turning of Eridanus (the River) lie a
seeming pair of fourth magnitude stars, Beid and
Keid, which Bayer
placed far down in the Greek alphabet as Omicron-1 and Omicron-2.
The part of Eridanus that contains Azha (Eta Eridani) and stars
south, where the River makes its second southerly plunge, were
known to the Arabs as the "Ostrich's Nest." Outlying Beid refers
to "the Eggs" and Keid to the "Egg Shells," all showing a wonderful
celestial mixture of cultures. Though to the eye they may look
like a real pair, they are very much not. Keid (Omicron-2) is very
close, a mere 16.5 light years away, where as Beid (Omicron-1) lies
over 7 times farther, 125 light years distant. To be roughly
similar in apparent brightness, Beid (mid-fourth magnitude, 4.04)
must be much the more luminous. Indeed, it is a white class F (F2)
giant, with a temperature of 7100 Kelvin and a luminosity 28 times
that of the Sun. Though direct measure of
mass is not possible (it has no known companion, whose orbit would
determine the mass), its current luminosity and temperature
compared with theory show its mass to be almost exactly double that
of the Sun. The star seems to have recently left the core-
hydrogen-fusing main sequence of stars, and is now evolving with a
dead (for now) helium core that is surrounded by a shell of fusing
hydrogen. With a radius of 3.5 times that of the Sun, its rather
high equatorial rotation speed of 100 kilometers per second show
the star to rotate in less than 2 days. Beid's real distinction,
however, is that it is a notable variable, a "
Delta Scuti" star,
the variation discovered only in 1971. Delta Scuti stars are less-
luminous versions of the Cepheid pulsating variables (those like Mekbuda). Unlike simple Cepheids, however,
they pulsate very subtly and rapidly with several periods at the
same time. Beid varies by at most only a few hundredths of a
magnitude (a few percent, invisible to the naked eye) with a
principal period of 1.8 hours, another of about 3.5 hours, and
certainly yet more. Such variations are extremely difficult to
study and are often quite uncertain. As Beid evolves to a cooler
giant, the instability will eventually cease.