LAMBDA CYG (Lambda Cygni). Just over the line into fifth magnitude (4.53), Lambda Cygni shines rather prominently about 2.5 degrees north of Cygnus the Swan's eastern wing, one of the two stars named "Gienah" (meaning "wing," the other in Corvus, the Crow or Raven), the star far better known as Epsilon Cygni. In the western branch of the Milky Way to the south of Deneb, Lambda Cyg falls to the west of the Milky Way's Great Rift, the central line of the Galaxy that is filled with obscuring dust. As a result, this blue-white class B (B5) dwarf is relatively undimmed even though 769 light years away (with about a ten percent uncertainty). Were the bit of dust not there along the line of sight, Lambda Cyg would be just about 0.2 magnitudes brighter. But calling it a class B (hydrogen- fusing) dwarf more than oversimplifies that star, as within a second of arc of one another are not one, but three stars (making it triple), with a possible fourth member a good distance away. With sophisticated interferometry, Lambda splits into two nearly identical stars, Lambda Aa (magnitude 5.4) and Ab (5.8) that orbit each other every 11.6 years at an average distance of 11.3 Astronomical Units, a fair eccentricity taking them between 17 nd 5 AU. Kepler's Laws then give a combined mass of 10.8 AU.
Lambda Cyg Aa Ab Lambda Cyg AB
At left, Lambda Cygni Ab and Aa closely orbit each other (note the scale, in seconds of arc) every 11.6 years at an average distance of 11.3 AU, brighter Aa at the cross, Ab seen as going around Aa. At right, Lambda Cyg B goes around AaAb with a 391 year period separated on average by 183 AU. The orbits are rather eccentric. The major axes of the true ellipses (dot-dash lines) and the orbital foci are offset from the apparent ellipses because of the orbital tilts to the plane of the sky. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.
With a surface temperature of 15,600 Kelvin Aa shines with the light of 1260 Suns, which leads to a radius of 4.9 solar and a mass of 5.5 Suns. Guessing a lower temperature for Ab we get 800 or so solar luminosities and a mass of 5.0, the sum very close to the orbital value, showing that parameters are close to correct. Lambda Cyg B, just under a second of arc away, is another story. At magnitude 6.3 (making it a somewhat cooler class B star), its principal values are 400 solar luminosities and about 4 solar masses. The long orbit of 391 years, a mean separation of 183 AU from the inner pair (going from 216 to 101 AU), yields lower accuracy, as witnessed by a calculated mass of 40 solar for the three, way off the mark. Clearly a longer observational baseline is needed. Way out, 83 seconds of are away, is Lambda Cyg C, which might be just a line of sight coupling. If not, it lies over 20,000 AU distant from the inner trio and takes more than 700,000 years to make a full orbit. That it is listed as a class K giant makes membership highly suspicious. The dominant star, Lambda Aa, seems to be an "emission-line" star, producing strong emissions from an encircling disk more or less edge-on, making it into a "shell star," though for such a star the projected rotation speed of 128 kilometers per second seems a bit low. Multiple systems like this one are common among the newly-formed hot stars of the Milky Way, Lambda Cyg giving us yet another fine example.
Written by Jim Kaler 7/27/12. Return to STARS.