OMI-1 CYG (Omicron-1 = 31 Cygni). It is hard to mention one of Cygnus's Omicrons without mentioning the other, especially given the confusion of names, whereby Flamsteed's 31 and 32 Cygni are sometimes called Omicron-1 and Omicron- 2, 30 and 31 Omicron-1 and Omicron-2, 30, 31, and 32 Omicron-1, Omicron-2, and Omicron 3. The only constant is that 32 Cygni remains Omi-2. Here we adopt the more modern view that 32 is Omi-2 as well, and let 30 shift for itself. Adding to the confusion, 30 Cygni (a class A dwarf) is sometime "related" to 31 Cyg as Omicron- 1 D, a false designation, since there seems to be no physical relation, just a line-of-sight coincidence in a crowded field of the Milky Way. That said, while none
Omicron 1 (the lower of the two bright stars) and Omicron 2 (upper) show the orange colors of class K giants. Just barely up and to the right of Omicron 1 lies contrasting white (almost blue in the photo) 30 Cygni, a class A5 dwarf. To the left and a bit up from Omicron 2 is a reddish star, U Cygni, a class R (carbon-type) Mira (long-period) variable.
of the Omicrons are related physically, Omi-1 and Omi-2 are connected through their phenomena -- that is, they are, by remarkable coincidence, that same kind of star. Both are "extreme Algols," or "Zeta Aurigae" stars in which a large cool giant or supergiant at least partially eclipses a hot blue class B dwarf that is in orbit around it. Many are the uncertainties connected with Omi-1. By itself, it is classified as a combination cool class K (a K2 bright giant) and as a hot B (B3) dwarf that comes in strongly in the blue and ultraviolet parts of the spectrum, the two known as Omi-1 A and B (and actually given two separate numbers in the classic Henry Draper Catalogue of stellar spectra). Shining at bright fourth magnitude (3.79), the K star would be itself be visual magnitude 4.3 and the B star 4.9. The two orbit each other with a long period of 3784.3 days (10.36 years), 3.3 times longer than the Omicron-2 pair. Given their separation of roughly 11 Astronomical units, it is remarkable that they are still aligned such that the K giant can still eclipse the blue dwarf to produce a drop of about a tenth of a magnitude, just barely enough to be seen with the naked eye, the eclipse lasting for 63 days. (The partial eclipse of the huge K star by the dwarf is not sensible.) The distance of the system is not well known, as the duplicity confuses the parallax. From the luminosity of the K star estimated through its class (about 1800 Suns) and an estimated temperature of 3900 Kelvin (to account for infrared radiation), the distance comes out to about 600 light years. (An unreliable parallax for the B star suggests twice as far.) The B star's luminosity should also be around 1800 solar, the K and B stars having respective masses of roughly 4-6 and 6-7 solar. That the evolved K star's mass is less than the unevolved B star's tells of the K star's severe mass loss. (Higher masses live shorter lives.) And like Omi-2, Omi-1 is surrounded by an huge extended atmosphere (which is eclipsed for some 75 days). The luminosity of Omi-1's cool star gives a radius nearly 100 times that of the Sun, while direct measure of angular diameter and the uncertain distance gives 120, actually not bad agreement. Not quite two minutes of arc away lies Omi-1 C, a class B5 dwarf whose apparent proximity is probably also a line-of-sight coincidence.
Written by Jim Kaler. Return to STARS.