IOTA LIB (Iota Librae). South of the famed third magnitude Claws of the Scorpion, Zubeneschamali and Zubenelgenubi, lies the dimmest of Libra's classical stars, fifth magnitude (though at 4.54 just barely) Iota-1 Librae. Immediately to the east lies sixth magnitude Iota-2, making the two look like a naked- eye double in the mold of Mizar and Alcor, or perhaps Epsilon Lyrae. Sadly, no, as Iota-2, a class A (A3) dwarf lies 240 light years away, much closer than Iota-1, which stands at a distance of 377 light years, the two just a line-of-sight coincidence. Iota-1, however, does not need Iota-2, as it is already a quadruple star on its own.
Iota Librae Iota Librae Ab goes around Iota Aa (the brighter and more massive of the two, placed at the cross) every 23.5 years at an average separation of 14.9 Astronomical Units. The points are individual observations to which the best orbit is fitted. The dot-dash line is the orbit's major axis. Since the orbit is tilted by 26 degrees to the plane of the sky, it is somewhat distorted from the way it would look if seen face-on. The arrow at lower right shows the direction of motion. Though Aa is placed at the focus of the Ab's orbit, in truth, both stars go about each other, orbiting around a common center of mass, which is not determined for this pair. North is down, as would be seen in a telescope. The scales around the edges are in seconds of arc. Far off the page is another pair (Iota B and C) that orbits this one. (From an article by B. D. Mason, G. G. Douglass, and W. I. Hartkopf in the Astronomical Journal, vol. 117, p. 1023, 1999, image courtesy of W. I. Hartkopf.)
Iota-1 was initially identified as a class A-peculiar star (with high silicon content), and then as a B9 subgiant-dwarf pair with magnitudes of 5.1 and 5.6 (but see below). Iota-1 Aa and Ab (as they are called) orbit each other with a period of 23.469 years at an average separation of 14.9 Astronomical Units (half again as far as Saturn is from the Sun), which at Iota-1's distance translates into a very difficult-to-separate 0.13 seconds of arc and that requires sophisticated instrumentation to split. A significant eccentricity takes them between 18.6 and 11.3 AU apart, "periastron" (the closest separation) last taking place in 1994, the next in 2018. Orbital period and separation then lead to a combined mass of 6.05 times that of the Sun. An estimated temperature of 11,000 Kelvin for both stars coupled with distance gives respective luminosities for Aa and Ab of 149 and 94 Suns, radii of 3.4 and 2.7 times solar, and masses of 3.1 and 2.9 Suns, which sum to almost exactly that determined through the orbit, showing that the various parameters are very accurate. Theory shows that both stars are also very clearly dwarfs about 60 percent of the way through their hydrogen-fusing lifetimes. Completing the whole set, not quite a minute of arc away lies another pair, Iota-1 B and C, respectively 10th and 11th more or less solar type class G (G4 and G8) dwarfs that are keeping pace with their larger class B mates (Aa and Ab), showing them to be real companions. The separation of Iota-1 B and C of two seconds of arc leads to a physical separation of at least 230 AU. Given subsolar luminosities and combined masses of 1.7 solar, the orbital period must be at least 2700 years. The G-dwarf pair is separated from the bigger B dwarfs (Aa and Ab) by at least 6600 AU, giving a period of at least 195,000 years. Given these various numbers to be accurate, from each of the pairs, the other would appear beautiful if not spectacular. From Aa or Ab, B and C would each roughly shine with the light of 10 times that of Venus separated by up to 2 degrees. A resident of B or C (and there is no evidence for any) would see the two B dwarfs each radiating the light of a full Moon up to 8 minutes of arc (0.13 degree) apart, the orbital movement quite wonderful to behold.
Written by Jim Kaler 5/09/08. Return to STARS.