ZETA CYG (Zeta Cygni). The sky's great bird, Cygnus the Swan, flies with outstretched wings to the south down the path of the Milky Way. Each wing is marked by one star, the northwestern one by Delta Cygni, the southeastern by Gienah (Epsilon). The latter wing, however, can also be stretched farther to third magnitude (3.20) Zeta Cygni (of no proper name), a "yellow" class G (G8) giant star. Shining at us from a distance of 150 light years from a surface with a well-determined temperature of 4980 Kelvin, Zeta radiates 119 solar luminosities into space. The radius of 14.7 times that of the Sun derived from temperature and luminosity agrees nicely with the value of 14.0 found from direct measure of the angular diameter coupled with distance, the two lying well within the errors of measurement of each other. The star's most likely status is that of a core-helium-fusing "clump giant" (so-called because of the large number of stars with similar characteristics that gather into a clump when graphed on a diagram of luminosity vs. temperature), though it might also be in transition to gianthood with a quiet helium core. If the former, the mass is around 3.0 solar with an age of about 400 million years; if the latter, the mass is a bit higher. Zeta Cyg, like Zeta Capricorni, Alphard, and several others, is also a mild "barium star" with a superabundance (about triple) compared to the Sun of that chemical element, as well as other heavy elements, in its atmosphere. Rather than making the elements themselves through nuclear reactions, such giants have been contaminated by once-more- massive companions that evolved first, made the stuff (by neutron capture deep in their nuclear-burning hearts), and then sent it on through mass-loss processes to their then-less-massive companions before turning into much lower mass white dwarfs (the lower mass the result of the same mass loss). And sure enough, Zeta Cyg has a faint companion. Spectroscopic observations reveal an orbiting star with a period of 6489 days (17.8 years). An assumption of a typical white dwarf mass yields an average separation of 11 Astronomical Units, the modest eccentricity taking the stars from as close as 8 AU to as far as 13 (some 30 percent farther than Saturn is from the Sun). And sure enough once again, Hubble Space Telescope observations reveal a little blip, the white dwarf itself, which is almost lost in the glare of the much brighter glow of Zeta Cyg itself.
The bright star is Zeta Cygni proper. Its tiny white dwarf companion, which was once the more massive star and a mighty giant that transferred some of its mass (and some heavy chemical elements) to Zeta (thus reversing the mass ratio), is the small blip at the lower left edge of the bright star's image. The two stars are here only 0.04 seconds of arc apart. From an article by M. A. Barstow, H. E. Bond, M. R. Burleigh, and J. B. Holberg in Monthly Notices of the Royal Astronomical Society. Hubble Space Telescope image courtesy of Howard Bond.
A detailed theoretical study shows that the system started with stars of 3.0 and 2.5 solar masses. The greater of these lost most of its mass to space in the act of becoming the white dwarf, sent half a solar mass to the lesser (including heavy chemical elements that it had manufactured), which increased the initially lower mass star to three solar masses and is the star we see today as Zeta Cyg. The system thus vividly demonstrates the dynamic binary interactions that can change the course of an individual star's history and fate.
Written by Jim Kaler. Return to STARS.