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.