CHI AND PSI PEG (Chi and Psi Pegasi), another two-for-one special. And this one is a natural for its rather remarkable symmetries. As Bayer neared the end of the Greek alphabet in his star-naming scheme for Pegasus (the Flying Horse), he arrived at Chi and Psi Pegasi, which straddle the eastern side of the Great Square (Alpheratz to Algenib), Chi to the south and east, Psi to the north and west, the two separated by six degrees (Chi almost on the Pisces border and notably closer to the Square's outline). The stars also straddle the equinoctial colure, the great circle that connects the equinoxes and the celestial poles, with Psi now the closer one of the pair to the colure. What is most remarkable is that that the two fifth magnitude stars (respectively 4.80 and 4.66) are both rather rare but quite similar class M (M2 and M3) red giants that appear to be in the same state of evolution. Chi is 368 light years away (give or take 9), Psi 476 (plus/minus 17), so they have no physical relation. With cool temperatures of roughly 3860 and 3590 Kelvin, Chi and Psi shine with the respective light (including a lot of infrared) of 440 and 1470 Suns, have radii of 47 and 100 times solar (if placed at the Sun would stretch halfway to Earth), and rather uncertain masses of around 2.5 to 3 Suns. The best guess is that both are brightening as giants for the second time with dead carbon/oxygen cores (the first brightening is with a dead helium core). Both are slight variables of a few hundredths of a magnitude and uncertain periods. As the more luminous, Psi appears the more advanced towards final expiration, when it will lose its outer envelope through a powerful wind (which nobody really understands), expose its hot inner core (which will light up the lost mass as a planetary nebula), and turn into a white dwarf of around 0.7 solar masses. Chi will soon follow. Now they diverge. Chi is moving at a fairly high pace relative to the Sun, 67 kilometers per second, over four times normal, whereas Psi is moving more leisurely at half that pace and in a different direction, emphasizing the stars' independence. Chi appears single, whereas Psi is binary.

Psi Peg Only a few observations determine the orbit of Psi Pegasi B about Psi Peg A (at the cross). In reality, each star is in orbit about a common center of mass that cannot be defined for this pair because of insufficient data. Given a distance of 476 light years, the fitted path suggests a separation of 25 Astronomical Units and a period of 55.1 years. Psi A does not appear at the focus of the fitted ellipse because of the tilt and orientation of the orbit, which does not appear face-on. (From the US Naval Observatory Double Star Catalog, courtesy of Bill Hartkopf.)

Its companion, of unknown nature but presumably small, is in a 55.1 year orbit with an average separation of 25 Astronomical Units, a modest eccentricity of 0.1 taking them between 22 and 27 AU apart. They were last physically closest in 2001 and will be again in 2056. Kepler's laws give a combined mass of 5.0 solar, a bit high, no surprise given a separation of just a couple tenths of a second of arc. Precession, the 26,000-year wobble in the Earth's axis caused by the gravitational pull of the Moon and Sun on our planet's rotational bulge, is dragging the Vernal Equinox to the west, which steadily increases the "right ascensions" of stars in this part of the sky (right ascension similar to longitude on Earth). The colure passed Chi Peg in 1718, making the star's right ascension jump from the 23-24 hour range back to 0 hours, the star thus beginning its cycle all over again. The colure will cross Psi in 2043. Party time.

Written byJim Kaler 12/19/14. Return to STARS.