RASALHAGUE (Alpha Ophiuchi).
Not far from a star with a similar sounding name,
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.