ETA CYG (Eta Cygni). Seemingly ordinary stars can offer nice surprises. On the bright side of fourth magnitude, nearly third (3.89), Eta Cygni is at times drawn in as part of the dotted line pattern that makes the long neck of Cygnus, the Swan (bright Deneb as the tail), or if you like the staff of the Northern Cross (with Deneb now on top). By itself, Eta Cyg is a classic orange class K (K0) giant in league with so many others of its kind. At a distance of 135 light years (with a remarkably low uncertainty of just 1) and a well-determined temperature of 4840 Kelvin (just right for its class), Eta Cyg shines with the light of 54 Suns, which leads to a radius of 10.5 solar. An indirect estimate of angular radius plus distance gives pretty much the same thing, 10.1 solar. The mass then comes in from theory at just under 2.5 Suns. With an age of about 750 million years, Eta is a stable helium burner that is converting its core to a mix of carbon and oxygen, the star having given up core hydrogen fusion perhaps 50 million years ago.

The importance of Eta Cyg lies not so much in what it is as it does in its surroundings. Embracing it is a flock of four "companions," 12.0 magnitude Eta B 7.8 seconds of arc away, 11th magnitude C (last seen 46 seconds away), 10th magnitude D (163 seconds), and 12th magnitude E (not quite a minute of arc). The motions of C, D, and E show them to be just "optical" line of sight coincidences. That leaves B, which may well be real. From its brightness, it's a class M0 red dwarf with a mass of about half a Sun. With a separation of at least 325 Astronomical Units, Eta Cyg B must take at least 3500 years to make a full circuit around Eta Cyg proper. From "B" the red giant would loom with the brightness of perhaps 175 full Moons, while from "A" the red dwarf would shine dimly with the light of a quarter Moon. Looking farther afield, Eta Cygni guides the eye (aided by a telescope) to the ninth magnitude binary X ray star Cygnus X-1, which lies just 7 minutes of arc to the north. It almost certainly harbors a massive black hole in orbit about a blue supergiant, the high energy radiation coming from a hot accretion disk around the black hole fed by the tidally distorted supergiant. In the other direction, you might find the rare class S variable Chi Cygni 2.5 or so degrees to the south southwest. The Mira type variable might be as bright as nearby Eta Cygni, and as such could almost be a part of the constellation pattern. But it might also be as faint as magnitude 14, well outside the range of binoculars, the variability cycle completed in about 400 days. In the distant future, after it has completely developed its carbon/oxygen core, Eta Cyg might behave somewhat similarly as it proceeds to slough off its outer envelope and die as a dense white dwarf.
Written by Jim Kaler 9/07/12. Return to STARS.