CUJAM (Omega Herculis). Allowing for some hyperbole, Cujam (ku-
yam, out of Latin) has more angles to it than a protractor. Not
hard to find in southeastern Hercules,
bright fifth magnitude (4.57) Cujam (Omega Herculis) lies between
the head of Serpens Caput to the west
and northern Ophiuchus to the east.
The corrupted proper name, which has
various spellings, is perhaps more grammatically served by "Caia."
In any case it refers badly to a club (with reference to Hercules)
and was affixed during the Renaissance. It's far better to use Bayer's designation of Omega
Herculis. The star thus has the dubious honor of holding up the
stack of the Greek letter system, which is topped in the constellation by Rasalgethi, Alpha Herculis (which lies
a dozen degrees nearly due east of Omega). If that's not enough,
as both 24 Herculis and 51 Serpentis, Omega Herculis is one of two
dozen stars given dual Flamsteed
numbers. While the latter is never now used, it shows the
uncertainty of early constellation boundaries (see Zeta Serpentis).
The star itself, a class B (B9p) "peculiar" presumed dwarf 250 light years away (give
or take 12), is a match for its name. The "p" refers to a weird
chemical composition caused by gravitational settling of some
elements and radiative lofting of others, one that is further
enhanced by a strong magnetic field. The prototype is Cor Caroli, Alpha-2 Canum Venaticorum in
Canes Venatici. Omega Her then becomes
an "alpha2 CVn star," one with especially large enhancements of
chromium and europium. Iron may be up significantly too. In such
a star the enhancements are concentrated into spots near the
rotation poles, whose axis is generally inclined to the line of
sight. As the spots rotate in and out of view, the stars vary,
Omega by a few hundredths of a magnitude over an interval of 2.961
days, which is taken as the rotation period. The projected
equatorial rotation speed of 46 kilometers per second, along with
radius (see below), gives 3.5 days. The rotation axis would then
be inclined to the line of sight by 63 or so degrees. Magnetic
fields up to several hundred times that of the Earth have been
measured. Distance, a 16 percent correction for dimming by interstellar dust, and a temperature
of 9730 Kelvin (to account for ultraviolet light) leads to a
luminosity 82 times that of the Sun and a
radius 3.2 times solar. Theory then points to a mass of 2.7 Suns
and shows that the star is indeed a dwarf, one about two-thirds of
the way through its hydrogen fusing lifetime of 450 million years.
After Omega Her's gianthood,
in which the core will eventually be fused to a mix of
carbon and oxygen and the outer layers ejected to make a planetary nebula, Omega Her will expire as a white dwarf with a final
mass of about 0.7 times that of the Sun. Three "companions" hover around. Two,
an 11th magnitude star (Omega C) 26 seconds of arc away and another
at 13th magnitude and 130 seconds (D), are from their motions
obvious line of sight coincidences. Eleventh magnitude (11.5)
Omega B, however, measured at 1.9 seconds away in 1878 and 0.8
seconds in 2000, may well be real. If so, it's a class K5 dwarf at
least 150 Astronomical Units from bright Omega A, and takes at
least a thousand years to orbit. Thanks to Latin scholar David
Bright for discussion of the name.
Written by Jim Kaler 8/22/13. Return to STARS.