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.