UNUKALHAI (Alpha Serpentis). Seemingly a true tongue-twister,
Unukalhai (oo-nook-ul-high), at bright third magnitude (2.65), is the
brightest star as well as the Alpha star of Serpens, the Serpent, the great snake that appears
wrapped around Ophiuchus, the Serpent Bearer. The name, closely
following the original Arabic, means "the serpent's neck," the star
more rarely called "Cor Serpentis," "the serpent's heart."
Serpens, representing the healing arts (see it on the physician's
staff), is the only constellation divided into two separate parts,
Unukalhai appearing in the western portion, Serpens Caput, the
Serpent's Head. (The eastern portion is Serpens Cauda, the
Serpent's Tail). With a temperature of about 4300 degrees Kelvin,
the star is a relatively warm class K (K2) orange giant some 15 times the solar
diameter across. No longer living off hydrogen fusion in its deep
core as does the Sun, the dying star is for a time most likely
illuminated by the fusion of helium into carbon and oxygen. At a
distance of 73 light years and typical of its class, it is to the
eye 38 times more luminous than our Sun.
If we add in the
invisible infrared radiation produced by the cool star, the
luminosity climbs to 70 times solar. Warm orange giants radiate X-
rays, while cool orange giants do not, the high-energy radiation
coming from activity vaguely like that produced by the solar
activity associated with sunspots. In league with its kind,
Unukalhai is a modest X-ray emitter. The star does stand out,
however, as an example of a "strong-lined giant," meaning that the
dark gaps in its spectrum, its rainbow of colors, are especially
strong, the result of a modest enhancement of metals and carbon in
its cool atmosphere.