TAU CYG (Tau Cygni). Appearing more or less anonymous within the Milky Way to the east of the main figure of Cygnus (the Swan), fourth magnitude Sigma (4.23) and Tau (3.75) Cygni make a modestly close pair about 1.5 degrees apart that provides a gateway to famed 61 Cygni (the first parallax star) about the same distance to the west of them and to the odd variable V1334 Cyg just to the east. But brighter Tau Cyg makes its own mark as a rather remarkable multiple star with nine named components (A through I), appearing almost as a small cluster. But we must admit that at least three of them (Tau Cyg C, D, and E, 72, 205, and 87 seconds of arc away) are moving too much relative to Tau proper ("A") and are clearly just line-of-sight coincidences. Twelfth magnitude Tau GH, a double in its own right, might be a victim too. Yet that still leaves a nice quadruple and a VERY well-observed inner double.

But first examine Tau itself, which is listed as a class F (F2) subgiant, but alternatively as an F0 or F3 dwarf. Subtracting the brightness of seventh magnitude (6.57) Tau Cyg B (an F7-G1 dwarf that lies just a second or so of arc away) dims Tau Cyg A to magnitude 3.83. From a rather nearby distance of 66.3 light years (give or take just a half) and with a well-defined temperature of 6660 Kelvin, Tau Cyg A shines with the light of 8.4 Suns, which leads to a radius of 2.2 times the solar value. Spinning with a projected equatorial speed of 96 kilometers per second, rather typical of the breed, Tau Cyg A then makes a full rotation in under 1.1 days and, probably related, shows some chromospheric activity. Theory then yields a mass of 1.55 Suns and clearly reveals the star to be a hydrogen-fusing dwarf mid-way through its allotted lifetime of 2.5 billion years. There is no chemical analysis to go along with a fairly high speed relative to the Sun of 48 kilometers per second, some three times normal (higher speed stars often having lower heavy element abundances).

Tau Cyg Seventh magnitude Tau Cygni B orbits brighter Tau Cyg A (at the cross, the two actually orbiting a common center of mass that lies between them) over a period of 49.62 years at an average separation of 18.5 Astronomical Units, which at the distance of the star translates into just 0.91 seconds of arc (the scale on the axes), making the binary a challenge to split in a small telescope. The well-defined orbit allows precise determination of the sum of the masses of the two stars. The distortion of the orbit (that Tau Cyg A is not at the focus of the ellipse) is the result of orbital tilt (46 degrees to the plane of the sky) and its orientation. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)

Close in, Tau B orbits Tau A every 49.62 years at an average distance of 18.5 Astronomical Units, a modest eccentricity taking them between 14.1 and 19.8 AU apart, the pair last closest in mid- January of 1989. If a G1 dwarf with a temperature of around 5700, it must be similar to our own Sun, but with a somewhat lesser luminosity. Application of Kepler's laws then gives a combined mass for the AB pair of 2.58 Suns, which is right on the sum of 2.55 derived from luminosity and temperature, showing that we understand the system well and providing yet another good check on theory. Two real additional companions are confirmed by Hipparcos satellite observations as tracking Tau AB through space. Ninety seconds of arc (at least 1770 AU) away lies twelfth magnitude (11.9) Tau Cyg F. From its brightness, it must be a class M2 or so red dwarf. With a mass of about 0.4 solar, it takes at least 43,000 years to orbit the inner AB pair. Then farther out at 534 seconds of arc (at least 10,800 AU) there is 16th magnitude Tau Cyg I, which from its brightness should be a low mass (0.08?) class M8 red dwarf, which would go about the inner trio in at least 650,000 years. Then there is the GH pair, 12th and 13th magnitude low mass probable M2-M4 dwarfs that are themselves separated by 9 seconds of arc and at least 170 AU, which would take more than 2800 years to go about each other, and if a part of the system would take at least 70,000 years to orbit AB. But it's a big "if" as the motion relative to AB is not defined. In spite of the uncertainties, as a curious multiple with a well-defined inner double, Tau Cyg no longer seems so anonymous.
Written by Jim Kaler 7/22/11. Return to STARS.