RASALHAGUE (Alpha Ophiuchi). Not far from a star with a similar sounding name, Rasalgethi in Hercules, second magnitude (2.08) Rasalhague is the luminary of the relatively dim but very large and intriguing constellation Ophiuchus, the Serpent Bearer. Attached to Ophiuchus's sides are the two non-contiguous parts of Serpens, the Serpent, the sky's only divided constellation. Ophiuchus represents Asclepius, the ancient Greek physician, the great snake embodied in the serpent- wound staff of the caduceus, the physician's symbol. Rasalhague itself embodies the constellation, the name coming from an Arabic phrase meaning "the Head of the Serpent Collector." From its measured distance of only 47 light years, we calculate that the star shines about 25 times brighter than our Sun. Like so many stars, it is not alone, but has a faint, very close companion only half a second of arc away that orbits with a period of 8.7 years. At Rasalhague's distance that separation corresponds to an orbit with a radius of 7 Astronomical Units, about halfway between the sizes of the orbits of Jupiter and Saturn. From the orbit, Rasalhague has a mass somewhere between 2 and 4 times that of the Sun, the companion much less, not much more than half a solar mass. More important and rather unusual, class A (A5) Rasalhague is just a bit brighter than the other stars of its temperature (about 8500 degrees Kelvin) and is actually classed as a giant star rather than main sequence star like the Sun. Rasalhague seems therefore to have recently run out internal hydrogen fuel and now is in the beginning stages of dying with a contracting helium core. It is also, similar to many of its class, slightly variable, though the effect is too subtle to be seen with the naked eye.
Written by Jim Kaler. Return to STARS.