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, in
1838 by Freidrich Bessel, the current best value a nearby 11.4
light years. Not only is the star double, but the two components
(not quite a half-minute arc apart) are among the dimmest ordinary
hydrogen-fusing dwarfs (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. 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."