CHI OPH (Chi Ophiuchi): A Tale of Two Stars. Chi and Kappa Ophiuchi could not be much more different. Kappa is a third magnitude cool orange class K2 giant star 92 light years away in the northern part of Ophiuchus (the Serpent Bearer), while Chi is a near-fifth magnitude (nominally 4.42) hot blue class B (B2) subgiant (but see below) at a vastly farther distance of 626 light years (give or take 18) buried deeply in the southern part of the constellation. What they have in common is that, written quickly, the Greek letters Kappa and Chi can look somewhat alike, which MAY have led astronomers to take Chi's variability (which runs about 0.8 magnitudes, roughly 4.2 to 4.8 over chaotic intervals measured in years) and transfer it to apparently stable Kappa, which was thought to be variable, but appears quite constant. That all said, the characteristics of Chi well trump Kappa, at least if you are looking for superlatives. First, Chi is distant enough that it is considerably dimmed by intervening interstellar dust. Were it not there, the star would appear 1.7 magnitudes brighter and shine at magnitude 2.7. After factoring in a large amount of ultraviolet light from its hot (20,900 Kelvin) surface, we find the star to radiate the light of 10,600 Suns, 200 times the power of Kappa. That figure leads to a radius of 8 times that of the Sun (where Kappa just barely wins with a radius 50 percent larger). Luminosity and temperature then tell of a star with a notable mass some 10 times that of the Sun, much greater than that of Kappa. With an age just short of 20 million years, Chi Oph appears to be an older dwarf, one that will soon give up core hydrogen fusion, so its designation as a subgiant -- a star preparing to become a giant -- is at least close to the mark. The star is right on or a bit above the line at which it will explode someday as a brilliant supernova.

Currently, however, Chi Ophiuchi is a "Be" star, one that radiates emissions from hydrogen that tell of a circulating ring of gas (the classic examples Gamma Cassiopeiae, Delta Scorpii, and Zeta Tauri). The large, long-term variation is produced by instabilities in the surrounding disk that cause it to change its characteristics, even its existence. If we take the magnitude at Chi's low state, the luminosity shrinks to 7500 Suns, the mass to 9 solar, and the age rises to 22 million years (with further uncertainty caused by errors in dust absorption produced by the ring). Most Be stars are rapid rotators. At 115 kilometers per second, Chi's seems rather small, which probably indicates that its rotation pole is rather pointed at us, the value thus a lower limit that yields a rotation period under 3.4 days. As are other Be stars, Chi is also a "non-radial pulsator," one that subtly pulsates and varies such that parts of the star move outward while other parts of the surface fall inward. The main period seems to be 0.65 days, with a variation of 0.03 magnitudes, which is superimposed on the large, longer-term variation. The pulsations are possibly related to the generation of the circulating ring, but nobody really knows what causes the Be-star phenomenon. Interestingly, Chi Oph is also double, with a spectroscopically-observed companion (about which nothing is known) that orbits every 139 days at a distance of at least 1.1 Astronomical Units, but with a high eccentricity that could take it as close as 0.6 AU to its mighty neighbor. More companionship comes from the "Upper Scorpius" association of O and B stars, to which Chi Oph firmly belongs. (Thanks to Phil Bagnall for suggesting this star and for providing information about the possible confusion with Kappa. Thanks also to Sebastian Otero, who provided a light curve and further commentary. See Kappa Oph for further references.)
Written by Jim Kaler 7/09/10; updated 7/30/10. Return to STARS.