RUCHBA (Omega-1 Cygni = 45 Cygni). There are so many stars in the sky that weird coincidences are almost mandatory. In northwestern Cygnus a few degrees more or less to the west of Deneb are two pairs of otherwise unrelated stars that each go by one of the two Greek versions of the Roman letter "O," the two Omicrons (Omi- 1 and Omi-2 Cygni) and, just three degrees to their northeast, the two fainter Omegas (Omega-1 and Omega-2 Cyg). The Omicrons are similar eclipsing Algol-like doubles, while the Omegas (just a third of a degree apart) are both visual doubles. Each pairs' names have been subject to considerable confusion, and each has a third embedded unrelated star to mess things up even more. The two Omegas have been confused with each other. Oddly, while the Omicrons have no proper names, dimmer Omega-1 (the brighter and western of its pair) is known as Ruchba (from an Arabic phrase meaning "the Hen's Knee), which is not be confused with the OTHER Ruchbah, Delta Cassiopiae, the two being distinguished only by their English spellings. Allen, in his famed book on star names, then takes the confusion even further by not only mixing the properties of Omega-1 and Omega-2 into his "Ruchba" but also by calling it "Omega-3"! Best to drop the proper name and to stick with Omega-1 (or its Flamsteed designation, 45 Cyg), a massive fifth magnitude (4.95) hot class B (B2.5) subgiant 910 light years away (give or take 45), more than twice as far as the class M red giant Omega-2. Lying within the Milky Way, the star is dimmed nearly half a magnitude by obscuring interstellar dust. Temperature measures, which are quite scattered, average 18,500 Kelvin, more appropriate to the alternative published class of B4 (bright giant). Using this value to account for ultraviolet radiation, we get a luminosity of 4800 Suns (5400 using the highest stated temperature of 19,500 K). A radius of 6.8 times that of the Sun and a projected rotation speed of 165 kilometers per second give it a spin period of under 2 days, typical for the class. Theory then indicates a mass between 7.5 and 8 times that of the Sun and shows it to be a true subgiant that has just given up core hydrogen fusion (or will soon). Very little is known about the star, except that it is NOT a Beta Cephei type variable, as it was once known. It's right under the mass limit at which stars explode as supernovae, and as such is expected to turn into a massive white dwarf. Some 17 seconds of are away lies thirteenth magnitude Omega-1 B, which from its brightness is roughly a solar-mass star that orbits at least 4800 Astronomical Units away and from Kepler's Laws over a minimum 8000 year period. Farther out, at 56 seconds distance (at least 15,500 AU) is ninth magnitude Omega-1 C, which is most likely a mid-class-A star that carries a couple solar masses and orbits the inner pair with a period greater than half a million years. Another "member," Omega-1 D, two minutes of arc away is, from its motion, probably just a line-of- sight coincidence. From "B," Ruchba proper would shine with the light of 20 full Moons, while in reverse, solar "B" would appear at around half a dozen Venus's from "A." From "A," Omega-1 C would notably outshine "B" even though much farther away.
Written by Jim Kaler 7/29/11. Return to STARS.