59 CYG (59 Cygni). Cygnus, the Swan, looks so serene as it glides down the Milky Way. The serenity is belied first by Deneb, one of the most luminous class A supergiants in the Galaxy, and by an additional host of massive neighboring stars that with Deneb make up the Cygnus OB7 association. A handful are bright enough to have Flamsteed numbers, among them 59 Cygni, a fifth magnitude (4.78) but variable (4.5-4.9) hot (22,550 Kelvin) class B (B1, probable dwarf) emission line (Be) star that lies 1420 light years away (give or take a whopping 250), about the same as Deneb itself. The spectral emissions tell of a surrounding disk similar to what we find around Gamma Cassiopeiae or Zeta Tauri. The disk appears to be rather in the line of sight, so seems especially thick to us, rendering 59 Cyg a special kind called a "shell star." Such stars behave erratically, 59 Cyg's disk more or less shutting down for a couple years in the mid-1970s. 59 Cyg is, to say the least, a complicated and confusing system with different components that are hard to disentangle. Considering just the visible star and nothing else, 59 Cyg suffers from three-fourths of a magnitude of dimming by interstellar dust. Factoring in distance, dimming, and ultraviolet light from the hot surface, we find a luminosity of 27,500 Suns, which translates to a radius of 11 times that of the Sun, a mass of 12 to 13 times solar, and suggests that the star is close to giving up core hydrogen fusion. But then there is the disk to contend with. It's related to 59 Cyg's remarkable 360 kilometer per second minimum equatorial rotation speed, which gives a rotation period of under 1.5 days. Moreover, the visible star has a very close, very hot (52,000 Kelvin) shrunken companion that orbits in just 28.2 days and heats the disk. It's probably the remnant of what was once a star of higher mass that died first, leaving the still-massive visible star behind, and it could have been tidally stripped by the star we see. Analysis of the whole system suggests a lower luminosity of about 8000 Suns for 59 Cyg proper and a mass perhaps 8 times solar. The hot companion comes in at 1000 solar luminosities and a mass of 0.8 Suns. The two orbit at about Mercury's distance from the Sun. Going around these with a period of 162 years is an 8th magnitude cooler class B, or even A, star some 0.2 seconds of arc away. The nominal average orbital separation is 90.5 Astronomical Units, a fair eccentricity taking them between 114 and 67 AU apart However, from Kepler's Laws the mass comes out too high, so the orbital radius is somewhat tighter, maybe 70 AU. It's hard to say whether or not 59 Cygni will explode as a supernova. Scattered around are four line of sight "companions" that have nothing to do with the system. Given the same distance, which is possible, Deneb would shine in 59 Cyg's sky with the light of our Venus. (Final summary from G, J. Peters et al. in the Astronomical Journal for March 1, 2013.)

Written by Jim Kaler 11/20/09; revised 11/15/13. Return to STARS.