OMEGA-2 CYG (Omega-2 Cygni = 46 Cygni). Two coincidental pairs of
stars grace northwestern Cygnus to the
west of Deneb, the two Omicrons and the
two fainter Omegas, the latter roughly 3 degrees northeast of the
former. Each has an embedded third fainter star to help confuse
the names, the stories told by Omicron-1
and -2 and by Omega-1 Cygni. Of the four (the Omegas and
the Omicrons), faint-fifth magnitude (5.44) Omega-2, 399 light
years away (give or take 9) and the eastern of the pair (46 Cygni
as opposed to Omega-1's Flamsteed
designation of 45 Cyg) is the faintest. None of the four (or
if you include the embedded stars, six) are related, all just
coincidental alignments, such not unexpected within the crowds of
stars in the Milky Way. Though having
nothing to do with each other, Omega-1 more than twice as far away
as Omega-2, they provide a lovely color contrast, Omega-1 a hot
blue class B dwarf, Omega-2 a red class M (M2) giant. While very much ignored,
with just 25 citations in the last century, the star is of interest
not only for its completing the quartet of Omicrons and Omegas, but
because it is a prime example of what is going to happen to our Sun. No one seems to have bothered to have
measured its temperature (needed to evaluate the amount of infrared
radiation), so we adopt 3700 Kelvin from the class, which with
distance gives a luminosity of 380 times that of the Sun and a
pretty decent radius 48 times solar, or just over half the size of
Mercury's orbit. Theory then shows Omega-2 to carry very close to
one solar mass, which gives it fine standing for showing our own
fate. That said, though, the exact state of the star is not fully
clear. It could be brightening as a red giant with a dead,
shrinking helium core, it could already have fired its helium to
fuse to carbon and oxygen and fading some, or it could be
brightening with a dead, shrinking carbon-oxygen core. Third-
option stars, however, tend toward variability, and Omega-2's
status as a variable is highly questionable (at most, five percent
(over a 30-day period). And since option one takes longer than
option 2, the former is most likely the case. With an age of 12.2
billion years, it gave up core hydrogen fusion 2.5 billion years
ago and still has a ways to go until the helium fires up, by which
time it will have reached a luminosity of 1000 Suns, three times
what it is today, and will have a radius almost as big as the orbit
of the Earth. Given that the Sun is now 4.6 billion years old, it
still has 7.6 billion years to go before it catches up with Omega-
2. If the latter had any inner planets, as
does the Sun, they are gone. Like its neighbor, Omega-1 Cyg,
Omega-2 has a companion, a
tenth magnitude star about a minute of arc away. If real, it is
another solar-type star (which would be odd, as it too should then
be evolving). However, the motion of five seconds of arc in nearly
two centuries relative to Omega-2 proper is much larger than would
be expected were the companion orbiting, so the "pairing" must just
be a line-of-sight coincidence, leaving evolving Omega-2 all alone.
Written by Jim Kaler 8/05/11. Return to STARS.