BETA SGE (Beta Sagittae). Sagitta, the Arrow, is yet another case of the common violation of the "rule of Alpha," that the brightest star gets the first letter of the Greek alphabet. Beta falls by the wayside as well, as the luminary is oddly third magnitude Gamma Sge, followed not by Beta but by bright-fourth magnitude Delta. Alpha and Beta then come in at a faint-fourth magnitude tie for third place (specifically both 4.37). Even odder, Alpha is the only star in the constellation with a proper name. Sometimes in his Uranometria, Bayer used position rather than brightness, but here even that rule is broken, as Delta falls between the Alpha-Beta pair and Gamma. But back now to Beta, which is a rather ordinary class G (G8) helium-burning giant, though one with a higher mass and bit of a composition anomaly. First though, from the star's distance of 440 light years (give or take 9), its temperature of 4860 Kelvin (to account for some infrared radiation), and a 20 percent adjustment in brightness as a result of dimming by interstellar dust, we find a luminosity of 429 times that of the Sun, which leads to a radius of 29 solar and (from a projected equatorial rotation velocity of 9 kilometers per second) a rotation period that could be as long as 160 days. Direct measure of angular diameter, though, gives a radius of 64 times that of the Sun, more than twice as great, for reasons unexplained, as most giant stars are far more well behaved with good agreement. Luminosity and temperature then give us a mass of around four times that of the Sun and an age of 130-140 or so million years. Though currently a "yellow giant" that is most likely quietly fusing its helium core into carbon and oxygen, Beta Sge started life as a blue-white mid-class-B dwarf, and after losing 80 percent of its mass though winds mostly after the death of the helium core, will end life as a 0.8 solar mass white dwarf (of singular nature, since there seems to be no stellar companion). While the iron content (relative to hydrogen) is right on solar, Beta Sge is mildly "cyanogen-rich." The cyanogen (CN) molecule is very common in the spectra of such coolish giants, and its elevation suggests that some fresh nitrogen has been moved upward from the stellar core by convection, that is, the star is slowly changing its external chemical composition. It would be fun to keep an eye on it if the processes did not take such a long time. As giants go, our Beta-of-the-Arrow also seems to have an especially turbulent outer atmosphere. The star's only other distinction is that it is useful as a comparison with which to watch the variations of neighboring Delta Sge, a double that contains a somewhat unstable class M red giant that shows us what Beta will someday be before it passes on as a white dwarf.
Written by Jim Kaler 10/01/10. Return to STARS.