PSI CYG (Psi Cygni). Toward Cygnus's western border with Draco, a bit over 10 degrees northwest of Deneb, lies (from Smythe rev. Chambers 1881) "A fine double star...A 5 1/2, bright white; B 8 lilac," the combined magnitude now settled at magnitude 4.92 (mid-fifth). The fainter of a close pair always seems to receive the nicer color, which we now know is just a proximity effect. In reality the stars are both white (but still a "pretty object"). First measured by William Herschel at a separation of 4.0 seconds of arc, the duo consists of a barely eighth magnitude (7.52) star going around a mid-fifth (4.96) magnitude class A (A4) dwarf. But that's not all! Closer interferometric scrutiny reveals Psi Cygni A (the brighter of the pair) to be broken into another double, rendering the star a nifty triple. Psi Cyg Aa, at magnitude 5.6, is coupled with Psi Ab (magnitude 6.1), the two just a tenth of a second of arc apart. From its brightness, Ab is probably an A7 dwarf, whilst the class of Psi B is listed all over the place, from A7 to F4; as a compromise, we'll adopt F0 (dwarf). Over the past two-plus centuries, the separation between Psi A and B has closed to 2.8 seconds of arc, which is not enough coverage for an orbit to be constructed. The inner pair, however, go around each other much faster. An orbit fitted to the positional data gives a period of 54.1 years and, at a distance from us of 281 light years (give or take 9), an average separation of 12.2 Astronomical Units. But something is amiss. At nearly 10 AU it takes Saturn some 30 years to orbit the Sun. The far more massive stars should take much less than 30 years to orbit, not more. Application of Kepler's laws to the orbital parameters yields a combined stellar mass for Aa and Ab of 0.6 solar masses, which is patently way out of line for class A or F stars. Given the small portion of the orbit observed, however, such a result is not at all surprising. A longer baseline, ideally the observation of a complete circuit, will settle the matter. So what do we have here? Aa is given a temperature of 8050 Kelvin, low for an A4 star, while from their classes we estimate 7800 for Ab and 7400 for Psi B. With little infrared or ultraviolet to deal with, the luminosities come in at 33 Suns for Psi Aa, 21 for Ab, and just six for Psi B, while radii are calculated to be 3.0, 2.5, and 1.5 times solar. Theory then gives respective masses of 2.2, 2.0, and 1.5 Suns. With the inner pair summing to 4.2 Suns, a period of 21 years, rather than 54, would be more like it. We'll just have to wait and see. A high projected equatorial rotation velocity of 256 kilometers per second applied to Aa gives a rotation period of under 0.6 days, but it's hard to know how contamination from Ab factors into it. At a separation from Psi A of at least 650 AU, B must take at least 700 years to orbit. From B, the inner pair might look like brilliant points of light separated by perhaps a degree, each radiating the light of a couple dozen full Moons. And yet more: fourteenth magnitude Psi C is 24 seconds away, eleventh magnitude Psi D nearly three minutes distant. These, however, are moving too fast to relative to Psi A to belong to the system, and alas are mere line of sight coincidences. We'll have to stick with a trio. (Thanks to Bill Hartkopf for instructive commentary.)

Written byJim Kaler 8/28/15. Return to STARS.