61 CYG (61 Cygni). Cygnus holds a seminal star, one not much appreciated within the classical figure that makes the celestial Swan and its famed asterism, the Northern Cross (the Swan tipped upside-down). Not only has it no proper name, it has no Greek letter name either, just a Flamsteed number, 61 Cygni. Yet anyone aware of the history of astronomy knows how important the star is, as it is the first one whose distance was measured (through its parallax) in 1838 by Freidrich Bessel, the current best value a nearby 11.4 light years (second Hipparcos reduction). Not only is the star double, but the two components (not quite a half-minute arc apart), with magnitudes of 5.2 and 6.0 (adding to 4.8), are among the coolest and dimmest ordinary hydrogen-fusing dwarfs (respective classes K5 and K7) visible to the naked eye. With temperatures of 4450 and 4120 Kelvin, they shine only at luminosities of 15 and 9 percent solar, their masses only 60 and 50 percent solar, radii just 65 and 60 percent solar. The two members of the system average 85 Astronomical Units apart, and take some 650 years to make a full turn around each other. As dimmer versions of solar type stars, the two have magnetic cycles similar to that of the Sun, the brighter 8 years, the fainter 11 years. As starspots (not actually visible) move across the stellar faces, the brightnesses vary, yielding rotation periods of 35 days. Magnetically active, each is also capable of popping a mighty "flare" that causes a sudden brightening. The pair is also known for its high "proper motion," its angular movement against the stellar background. Though some of this high motion comes from the stars' proximity to us, they are also clipping along at a healthy pace relative to the Sun, at 108 kilometers per second, over five times faster than the Sun moves relative to its neighbors, implying that 61 Cyg is not a member of the thin disk of our Galaxy, but is merely a visitor to the neighborhood. For its place in history, 61 Cygni is included in "The Hundred Greatest Stars."
Written by Jim Kaler 8/07/09; updated 8/07/09. Return to STARS.