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.