68 CYG (68 Cygni). Number 68 of Cygnus
(the Swan) may seem like just another dim fifth magnitude (actually
5.00, right on the mark) Flamsteed star. It isn't.
Instead it's one of the more massive stars to be found in the Galaxy, a rare class O (O8) dwarf some seven degrees east-
southeast of Deneb in the heart of the Milky Way just four degrees off the Galaxy's
centerline. Take away almost a magnitude (0.93 mag) of dimming by
interstellar dust, and the star
would not only brighten to mid fourth magnitude, but would have its
blue-white sparkle restored, the true color the result of a
whopping surface temperature of 34,800 Kelvin. Not that a single
defining temperature is possible, since the star is a remarkably
fast rotator with an equatorial spin speed of at least 317
kilometers per second, which flattens it some, resulting in a
higher polar temperature and a lower equatorial one. 68 Cyg is so
far away that its distance is only poorly known. Direct parallax places it 4660
light years away, but with a formal uncertainty that puts it
anywhere from 3500 to 6900 light years. Listed as belonging with
Deneb to the Cygnus OB 7 association, it seems to be too
far away given Deneb's distance of around 1400 light years. With
a speed relative to the Sun (mostly across
the line of sight) of 66 kilometers per second, four times
"normal," the star may be a runaway from somewhere else. Assuming
the distance to be correct, and allowing for a very large amount of
ultraviolet radiation from
the hot surface, we find an amazing luminosity of 832,000 times
that of the Sun, which leads to a radius of 26 times solar and a
rotation period of under 4 days. To get the same irradiance as we
do from the Sun, a hypothetical Earth would have to orbit 900
Astronomical Units away, almost 25 times Pluto's distance from the
Sun. Theory then suggests a mass that might be as high as 60 Suns,
making 68 Cyg one of the most massive stars of the Galaxy. High
mass stars fuse their interior hydrogen at such a fast pace that
they do not live very long, 68 Cyg perhaps three-quarters of the
way through its dwarf lifetime of 3.5 million years (by contrast
the Sun's is 10 billion years). Even taking just the lower limit
of distance gives nearly half a million solar luminosities and a
mass of 45 solar. Various research studies place the mass between
26 and 50 Suns. The star however may have a contributing binary (spectroscopic) companion in a 5.1
day circular orbit. Since its spectrum is unseen, its effect on
the calculated stellar mass, while probably minimal, is not known.
The orbit gives 68 Cyg the reputation of being an "ellipsoidal
variable" as different cross sections of a tidally distorted star
present themselves to us. Emission
lines in the spectrum tell of a high speed wind blowing at up
to 2300 kilometers per second at a mass-loss rate more than 10
million times that of the solar wind, in league with all developing
O stars. In a wonderful setting, 68 Cyg is centered on a vast diffuse emission nebula more than
100 light years across, which seems to be illuminated by it.
Whatever the details, there is little question that 68 Cygni is a
prime example of a star that is going to blow up big time as a supernova, one that given the
right set of parameters might even create a black hole.
Written by Jim Kaler 11/22/13. Return to STARS.