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.