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.
Written by Jim Kaler. Return to STARS.