KAPPA CYG, IOTA CYG, AND, WHY NOT, IOTA-1 CYG (Kappa, Iota, and Iota-1
Cygni, a three-for-one-special.) Separated by 2.7 degrees, Kappa and
Iota Cygni lie in northwest Cygnus
(the Swan) near the border with Draco,
Kappa actually Flamsteed 1,
Iota 10 Cygni. Look somewhat under 10 degrees north-northwest of Delta Cyg. They are a curious pair of
almost identical fourth magnitude brightness (respectively 3.77 and
3.79). Just under a degree northwest of Iota is sixth magnitude (5.75)
7 Cygni, which also goes by the name Iota-1, plain Iota then being
Iota-2. The name is rarely used, however, Iota-2 usually called just
"Iota." Star names do get messy.
Though Kappa and Iota are very different beasts -- Kappa is a class
G (G9) giant, Iota a class A (A5)
main-sequence dwarf -- they
are even more curiously at almost identical distances: Kappa is 124.2
light years away (give or take 0.5), while Iota is 122.3 light years
(with a formal error of just 0.3) from us. Factoring in a fair bit
of infrared radiation from a
surface with a well-defined temperature of 4990 Kelvin, Kappa Cyg
shines with a luminosity of 47.4 times that of the Sun, which leads to a radius of 9.2 times solar,
small for a giant. An interferometer measurement of angular diameter
gives 8.5 solar radii, nine percent smaller (with no idea why except
that giants can have fuzzy edges). Theory yields a mass 2.5 times that
of the Sun and an age of around 600 million years. On the other hand,
Iota's temperature is not well defined at all, two listings ranging
from 7300 to 8260 Kelvin with a mean of 7880 Kelvin. Iota's luminosity
of 32.4 Suns (nearly all of which is in the visual spectral domain)
gives us a radius of 3.2 times solar and a somewhat lower mass of 2.2
Suns. Kappa is an ordinary core-helium-fusing giant, while Iota is
at near the end of its hydrogen-fusing life of around a billion years
and (the mass difference aside) will before long turn into a giant
rather like Kappa, the two nicely illustrating how one kind of star
transforms itself into another. As are so many stars in its class,
Iota is a rapid spinner with a projected equatorial velocity of 229
kilometers per second, resulting in a rotation period of under 16.6
hours. Rapid rotation results in a distorted star with a notable
equatorial bulge, which may be the cause of the temperature problem,
as stars are cooler at their equators than they are at their poles (an
effect called "gravity darkening"). Kappa and Iota Cyg even have
rather similar motions. Each
is coming toward us at respective speeds of 29 and 20 kilometers per
second, and each is speeding across the line of sight in sort of the
same direction at 25 and 23 km/s. Separated by 6.2 light years they
are clearly not gravitationally bound, yet their similarity of motion
suggests a common origin in the same, now long-gone, birth cloud.
Are there others? Certainly not Iota-1 (woops, 7) Cygni, as it's 351 light
years away (with an uncertainty of 9), three times farther than Kappa
and Iota. A class A1 (nearly "colorless") dwarf with a temperature
of 8830 Kelvin, 7 Cyg radiates at a rate of 48 Suns, yielding a radius
of 3.0 times solar and a mass of 2.4 Suns, the star in the latter half
of its hydrogen-fusing lifetime of 630 million years. With a lower
projected equatorial rotation speed of 52 km/s (which could of course
just mean that the star's rotation axis is significantly tilted to the
line of sight), Iota-1 could take as long as 2.9 days to make a full
rotation. None of the three stars has any trace of a companion, all
seemingly single. Whether there are planets remains unknown. Iota
does not even seem to have an infrared-radiating debris ring, while
Iota-1 Cyg has not been examined. If someone were on either Kappa or
Iota, each would shine at magnitude -2.7 in the other's sky, three times
the brightness of Sirius in ours and roughly similar to that of Jupiter.
Written byJim Kaler 9/11/15. Return to STARS.