59 CYG (59 Cygni). Cygnus, the Swan,
looks so serene as it glides down the Milky
Way. The serenity is belied first by Deneb, one of the most luminous class A
supergiants in the Galaxy, and
by an additional host of massive neighboring stars that with Deneb
make up the Cygnus OB7 association. A handful are bright
enough to have Flamsteed
numbers, among them 59 Cygni, a fifth magnitude (4.78) but
variable (4.5-4.9) hot (22,550 Kelvin) class B (B1, probable dwarf) emission line (Be) star
that lies 1420 light years away (give or take a whopping 250),
about the same as Deneb itself. The spectral emissions tell of a
surrounding disk similar to what we find around Gamma Cassiopeiae or Zeta Tauri. The disk appears to be rather
in the line of sight, so seems especially thick to us, rendering 59
Cyg a special kind called a "shell star." Such stars behave
erratically, 59 Cyg's disk more or less shutting down for a couple
years in the mid-1970s. 59 Cyg is, to say the least, a complicated
and confusing system with different components that are hard to
disentangle. Considering just the visible star and nothing else,
59 Cyg suffers from three-fourths of a magnitude of dimming by interstellar dust. Factoring in
distance, dimming, and ultraviolet light from the hot
surface, we find a luminosity of 27,500 Suns, which translates to a radius of 11 times
that of the Sun, a mass of 12 to 13 times solar, and suggests that
the star is close to giving up core hydrogen fusion. But then
there is the disk to contend with. It's related to 59 Cyg's
remarkable 360 kilometer per second minimum equatorial rotation
speed, which gives a rotation period of under 1.5 days. Moreover,
the visible star has a very close, very hot (52,000 Kelvin)
shrunken companion that orbits in just 28.2 days and heats the
disk. It's probably the remnant of what was once a star of higher
mass that died first, leaving the still-massive visible star
behind, and it could have been tidally stripped by the star we see.
Analysis of the whole system suggests a lower luminosity of about
8000 Suns for 59 Cyg proper and a mass perhaps 8 times solar. The
hot companion comes in at 1000 solar luminosities and a mass of 0.8
Suns. The two orbit at about Mercury's distance from the Sun.
Going around these with a period of 162 years is an 8th magnitude
cooler class B, or even A, star some 0.2 seconds of arc away. The
nominal average orbital separation is 90.5 Astronomical Units,
a fair eccentricity taking them between 114 and 67 AU apart However,
from Kepler's Laws the mass comes out too high, so the orbital
radius is somewhat tighter, maybe 70 AU. It's hard to say whether
or not 59 Cygni will explode as a supernova. Scattered around
are four line of sight "companions" that have nothing to
do with the system. Given the same distance, which is possible,
Deneb would shine in 59 Cyg's sky with the light of our Venus. (Final summary from G, J.
Peters et al. in the Astronomical Journal for March 1, 2013.)
Written by Jim Kaler 11/20/09; revised 11/15/13. Return to STARS.