Alpha Ophiuchi - Rasalhague

At magnitude 2.08, Rasalhague is the brightest member of the constellation Ophiuchus, the Serpent Bearer. Rasalhague embodies the constellation, its name coming from an Arabic phrase meaning "the Head of the Serpent Collector."

Properties

From a distance of only 47 light years, the star shines about 25 times brighter than our Sun. It has a faint, very close companion only half an arcsecond 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 AU. From the orbit, Rasalhague has a mass somewhere between 2 and 4 times the Sun's; the companion's is much less, not more than 0.5 solar masses.

Class A5 III Rasalhague is brighter than most other stars of its temperature (about 8500 K), and is actually classed as a giant star. Rasalhague seems to have recently run out internal hydrogen fuel, and now is in the beginning stages of becoming a red giant with a contracting helium core. It is also - similar to many stars of its class - slightly variable, though the effect is too subtle to be seen with the naked eye.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]