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.