IOTA PEG (Iota Pegasi). Pegasus, Perseus's Flying Horse, famed of the Andromeda myth, is best known for its Great Square. But there is far more to the constellation. Each of the Square's four stars but Algenib (Gamma Pegasi) serves as an anchor for streams of other stars that contain their own treasures. At the top of the list, the northeastern star of the Great Square, Alpheratz, is the foundation for Andromeda (the star, Alpha Andromedae, actually belonging to the Maiden), while off the southwestern corner, Markab (Alpha Pegasi) starts the hooked line that leads us to Enif (Epsilon Peg) and the bright, very compact globular cluster Messier 15. Coming off Scheat at the Square's northern apex is a spray of stars that directs the eye to our star, bright-fourth-magnitude (3.76) Iota Pegasi, and even onward to fainter but similar Kappa Peg. A seemingly ordinary mid- class-F (F5) dwarf, Iota Peg is one of the closer stars to the Sun, lying just 38.3 light years away (give or take only 0.3!). It appears first as an ordinary visual double, with 11th magnitude Iota Peg B last seen 126 seconds of arc away. Rapid relative motion (34 seconds of arc over a century and a half, consistent with Iota's angular movement against the background stars), however, clearly shows "B" to be just a line-of-sight coincidence. But then we look deeper to find that Iota A is indeed binary, but instead a close spectroscopic double with absorptions of both stars visible, the two going around each other in a nearly circular orbit with a very short period of a mere and remarkably accurate 10.213033 days (10 days, 5 hours, 2 minutes, and 53 seconds).
Iota Peg Iota Pegasi B goes about Iota Peg A (at the cross) in a precisely- known path (the two actually orbiting a common center of mass between them) every 10.21 days. The nearly circular orbit looks elliptical because it is so highly tilted, through an angle of 77.6 degrees, to the plane of the sky. The scale is in seconds of arc; note how close the stars are to each other. (From W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)
Iota Pegasi, along with a handful of others, is outstanding for the precision of its characteristics. From a combination of the velocities of the two components, interferometer observations of positionings, and orbital mechanics, astronomers find amazingly accurate masses of 1.324 and 0.825 solar masses, the stars separated by just 0.119 Astronomical Units, 30 percent Mercury's distance from the Sun. Such data provide excellent checks on stellar structure theory. From Iota A's temperature of 6510 Kelvin, ignoring the companion for now we estimate a luminosity of 3.4 times that of the Sun and a radius of 1.45 times solar, consistent with an indirect estimate from the literature of 1.39. Theory then gives a mass of 1.3 Suns, just that derived from the orbital data. The mass of the companion, Iota Ab, tells that it is probably a class K0 dwarf, while a temperature estimate of 4990 Kelvin is consistent with K3, the spectrum itself suggesting G8. Taking the companion's brightness into account lowers the luminosity of the main star, Iota Aa, by maybe 10 percent, but has little influence on the derived mass. The metal content is close to solar, while a projected equatorial rotation speed of 7.5 kilometers per second gives a rotation period for Iota Aa of less than 9.6 days, not far from being synchronized with the orbital period. Ab, though, appears to be spinning a bit too fast for synchrony. (Data from a paper by B. B. Behr et al. in the Astronomical Journal vol. 142:6, July 2011.)
Written by Jim Kaler 10/28/11. Return to STARS.