STAR OF THE WEEK: PSI SGR (Psi Sagittarii). The "Pointers" of the Big Dipper in Ursa Major are famous for directing the eye northwards toward Polaris, while those of Corvus point eastward towards Spica in Virgo. The counterpart stars in Sagittarius, those of the Little Milk Dipper, sadly don't point toward much of anything except the blanket of stars of the Milky Way, But if you run them backwards three of four degrees toward the northwest, you encounter fifth magnitude (4.85) Psi Sagittarii, which also lies about five degrees west of the constellation's second brightest star, second magnitude Nunki, Sigma Sgr. Why, when Polaris is so important a navigational star, and Psi Sgr can barely be picked out of the stellar morass of the Milky Way, would anyone care about it? Supposedly it depends on how much one likes true triple stars. It's not an easy task to separate them. Psi Sgr first looks like a very close double with components separated by just a fraction of a second of arc, wherein one of the two is also a closer binary as determined by spectral analysis. This sort of setup is the only way a triple star can be stable. If you put the stars at comparable distances from one another, an inevitable fast close encounter between two of them will kick one out of the system at high speed and we wind up with a close double and a runaway star. For stability, we need a tertiary star far enough from an inner binary such that the tertiary "sees" the binary as a single star or gravitational point. Then the trio can be together practically forever. Psi Sgr, 298 light years away (with an uncertainty of about 9), may set some kind of record for attempts to establish the spectral classes as the light of the three blends together. You can find anything from class K to A. Apparently the outer star is a class G (G8) giant, while the inner pair consists of an A9 giant and an A7 dwarf.

Psi Sgr The outer G giant mutually orbits the two inner class A stars of the triple star Psi Sagitarii on aighly eccentric path over a 20-year period, the outer star and the close binary averaging 12 Astronomical Units apart. In spite of the obviously large errors for individual observations, the sum of the masses from Kepler's laws comes out quite reasonably at nearly eight Suns. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

The inner, close, pair orbits every 10.78 days. Assuming published masses of 2.3 and 1.7 Suns, they average 0.15 AU apart, less than half Mercury's distance from the Sun. These two then mutually orbit the class G giant every 19.95 years. An established (if crude) orbit makes them 12.08 AU apart, a rather high eccentricity ferrying the stars between seven and eighteen AU at the extremes, the whole system nicely satisfying the criterion for a stable triple. They were last closest near the very end of the year 2014. Kepler's laws give a total mass to the trio of 7.7 times that of the Sun, which, given the large orbital uncertainties, is surprisingly close to the 6.9 or so solar masses from published values. From the outer star, which seems to be a dying helium-fusing giant, the inner pair would be a blindingly bright duo at most three-quarters of a degree apart. The outer one will be the first to slough its outer envelope and perhaps produce a planetary nebula with the old nuclear-burning core turning into a white dwarf. As the inner pair evolves, the individuals may well merge as they expand as giants, then produce another planetary nebula, leaving us finally with a simple double instead of the triple star we see now. (Thanks to K. G. Strassmeier and F. C. Fekel in Astronomy and Astrophysics, 230, 389, 1990, for their summary of the stellar characteristics.)
Written byJim Kaler 11/04/16. Return to STARS.