GAMMA PHE (Gamma Phoenicis). The majority of evolving stars are rather settled giants of classes G and K, which are quietly fusing their core helium into carbon and oxygen. Here and there though one finds a redder class M giant that, far from being settled, is in some kind of transitional state, though exactly what state may not be immediately evident. At the northern tip of the southern constellation Phoenix, the Firebird, is second magnitude Ankaa, Alpha Phoenicis, in brightness followed by third magnitude Beta, which are respectively typical K and G giants. To the east of Ankaa however, is third-ranked third magnitude (3.41) Gamma Phe, the rarer class M (M0) giant that is our focus. At a distance of 235 light years, the star radiates at a rate 575 times that of the Sun, most of its light coming out in the infrared, as the star's surface is rather cool, only 3900 Kelvin -- an estimate based on class, as oddly there are no actual measurements of it. Like most such stars, Gamma Phe is large, its temperature and luminosity giving a radius 53 times solar, a quarter of an Astronomical Unit (AU), 65 percent the size of Mercury's orbit. The theory of stellar structure and evolution yield a mass only 25 percent greater than that of the Sun, so the star gives some sense of what is going to happen to us sometime in the next several billion years. Having a somewhat greater mass, though Gamma is evolving faster than will the Sun, and is now a bit over 5.5 billion years old -- the Sun has almost that much time still left to it as core-hydrogen burner. The actual status of Gamma Phe is arguable. It might be brightening with a dead helium core or it might have just started fusing helium to carbon and is dimming as it prepares to settle in as a quiet class K giant. However, Gamma Phe is also a semi-regular or irregular variable star that erratically (there is no known period) changes between magnitudes 3.39 and 3.49, which marginally implies that it has ceased helium fusion and is brightening for the second time, now with a quiet carbon core. Far more certain is that the star has a companion (revealed spectroscopically) with a period of 193.85 days. Nothing else is known about it. If it is a low mass dwarf, the two average just over 0.65 AU apart, whereas if it is as massive as the Sun, the companion is 0.8 AU from its redly-glowing, dying mate, which is destined to slough off its outer envelope and become a shrunken white dwarf.
Written by Jim Kaler. Return to STARS.