Beta Ophiuchi, of magnitude 2.77, lies at the northeastern corner of the pentagon that makes up the classic figure of Ophiuchus, the Serpent Bearer. It has the traditional name Cebalrai (alternately Cheleb, Kelb Alrai, or Alrai) meaning "shepherd's dog" in Arabic. These names come from an ancient Arabic constellation that stretches from Hercules to southern Ophiuchus, and represents a pasture. The Arabs visualized α Oph as a shepherd, along with his dog and some sheep, in this area.
Cebalrai is known for its setting: it makes a guidepost to the old, no-longer-recognized constellation "Poniatowski's Bull" (a V-shaped figure just to the east of the star), and to the large open star cluster IC 4665 (just to the northeast).
Properties
Physically, Cebalrai is a relatively cool (4600 K) orange class K2 III giant. At a distance of 82 light years, Cebalrai radiates 64 solar luminosities into space, from a sphere with a radius 12.5 times that of the Sun. The star has a mass of about twice the Sun's. Having long ago finished hydrogen fusion that powers main-sequence stars like the Sun, it is now fusing helium into carbon in its deep core.
Variability
Like some other K-type giants, β Oph has been found to vary slightly (0.02 magnitudes) in brightness. Closer physical examination reveals Cebalrai to be variable in size. Doppler measures show the star to jitter with periods of 0.26, 13.1, and 142 days. The longer period is caused by rotation, and is consistent with a rotational velocity of 2 km/sec. The 13-day period, however, seems to be a real - though quite subtle - pulsation, whose origin is not well understood. A similar kind of pulsation has also been discovered to take place in Arcturus.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]