At magnitude 2.65, Alpha Serpentis is the brightest star in Serpens, the Serpent, which appears wrapped around Ophiuchus, the Serpent Bearer. Serpens represents the healing arts - see it on the physician's staff - and is the only constellation divided into two separate parts. Unukalhai appears in the western portion, Serpens Caput, the Serpent's Head. The eastern portion is Serpens Cauda, the Serpent's Tail.
Alpha Serpentis has the traditional name Unukalhai (alternately Unukalhay, Unuk al Hay, Unuk Elhai, Unuk), from the Arabic "unuq al-hayya", meaning "the serpent's neck". The star is more rarely called Cor Serpentis, a Latin phrase meaning "the heart of the serpent".
Properties and Companions
With a temperature of about 4300 degrees K, α Ser is a cool class K2 III orange giant some 15 times the Sun's diameter. The dying star is powered by the fusion of helium into carbon and oxygen. At a distance of 73 light years, it 38 times more luminous than our Sun visually; if we add infrared radiation, the luminosity climbs to 70 times solar. Unukalhai is a modest X-ray emitter; the high-energy radiation coming from activity like that associated with sunspots. The star is a "strong-lined giant"; the dark lines in its spectrum are especially strong due to an enhancement of metals and carbon in its cool atmosphere.
Alpha Serpentis is a triple star system, with two optical line-of-sight companions. The magnitude +11.8 α Ser B is 58" from the primary, and the 13th magnitude α Ser C lies 2.3' from A.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]