GAMMA CRA (Gamma Coronae Australis). The modern constellation of Corona Australis, the Southern Crown, is fainter than its northern counterpart, Corona Borealis, but is nevertheless easy to find due south of the Little Milk Dipper of Sagittarius and east of the tail of Scorpius. The top brightest stars, Alpha through Epsilon, are a bit unusual as they actually line right up in order of brightness, Alpha (the luminary at 4.11, tied with Beta), then fourth magnitude (4.35) Gamma, followed almost immediately by Delta and Epsilon. Bayer was supposed to have worked this way, but often did not. Of the five (and there are many fainter ones that complete the constellation), Gamma is just barely the most northerly of them. At first it looks like a bit of a cheat, since Gamma is not one star, but two, an orbiting binary made of two mid-temperature class F (F8) dwarfs of magnitudes 4.53 and 6.42. Even removing the companion, however, renders Gamma CrA in third place. Not just a binary, it's a visual double whose first measure of separation was made by John Herschel (son of William) in 1834, who called it "superb." The stars are currently 1.4 seconds of arc apart. Early on they were suspected of orbital movement, and by now we have a complete observed orbit wherein the stars go around each other with a period of 121.76 years. The distance to the binary was not observed by Hipparcos, so we must use alternative measures.

Gamma Cra With a well-defined orbit, Gamma Coronae Australis B goes around Gamma A (at the cross) every 122 years (in reality both orbiting a common center of mass roughly halfway between them) averaging some 33 AU apart. Unfortunately the distance to the system is not all that well known, so the results are a bit problematic. (From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.)

An uncertain distance of 56 light years (give or take 0.8?) yields a mean separation of 32.8 Astronomical Units, a fairly high eccentricity bringing them as close together as 22 AU and as far apart as 43 AU. They were last physically closest (though not necessarily closest as seen on the sky) in mid-2000 The classes are a bit problematic, as the fainter has been listed as cool as G1, similar to out G2 Sun. Kepler's Laws applied to the orbital parameters gives a combined mass of 2.4 times that of the Sun. With a temperature of 6188 Kelvin and thus little infrared radiation to add to the starlight, Gamma CrA A has a luminosity about 3.7 times that of the Sun, which gives it a radius of 1.7 times solar, and a mass near 1.25 Suns. Gamma CrA B, however, can't really be analyzed as it can't be fitted to theory. The problem seems that the class and temerature are just not known. If we simply say that it too carries 1 1/4 solar masses, then we come up with a sum of 2.5 Suns, almost exactly that derived from the orbit. If The projected equatorial rotation speed is 7.6 kilometers per second, Gamma CrA A rotates with a period less than 11 days. Whatever the details, both stars are beneath the "rotation break" near class F5, above which (hotter than) main sequence stars spin faster as a result of the loss of their outer convection zones and magnetic fields, the latter acting to slow the stars down as they are dragged out by the stellar wind. The stars ae too far apart to affect each other during later evolution, and the system will probably die as a double white dwarf.
Written byJim Kaler 11/18/16. Return to STARS.