PHI AQR (Phi Aquarii). Want to see what will someday happen to the Sun? Here is a fine chance, from a star that lies within the constellation of Aquarius, the "Water Bearer." Phi Aquarii (with no proper name) is a fourth magnitude (4.22) class M (M1.5, sometimes called M2) red giant that lies a dozen degrees southeast of Aquarius's famed "Water Jar" and a bit less to the south of the western end of Pisces' "Circlet." Just a degree off (to the south of) the ecliptic, a line drawn from it to Lambda Aquarii more or less parallels the apparent solar path. Rather well-observed, the temperature from various studies is pretty consistent around the average of 3760 Kelvin, the star producing much of its radiation in the invisible infrared. When that is accounted for, and Phi's distance of 202 (give or take 9) light years is factored in, we find a luminosity 265 times that of the Sun and a radius of 38.5 solar, which is about half the size of Mercury's orbit. Since Phi Aqr is close to the ecliptic, it's regularly occulted (covered over) by the Moon. From the time it takes the star to disappear behind the lunar disk, and knowing the rate at which the Moon moves across the sky, the star's angular size is readily found. Distance then leads to a smaller physical size of 32.7 solar radii. (The errors of measurement, however, are such that the two values are consistent with each other.) The projected rotation is so slow that it has never been measured, but it could be as long as 1.3 years. The real interest, though, is that when compared with calculations from the theory of stellar structure and evolution, Phi Aquarii has almost exactly the luminosity and temperature expected for the Sun after a life of 12.2 billion years since birth (7.6 billion years from now). Having given up core hydrogen fusion 2.4 billion years ago (at an age just shy of 10 billion years), Phi Aqr is now most likely brightening as a red giant with a dead helium core, which will shortly begin to fuse into carbon and oxygen, the star then shrinking and dimming a bit to become a quiet class K giant helium burner of the sort that flocks the sky. At its coming peak, Phi Aquarii will reach a luminosity of nearly 1300 solar and a radius half the size of Earth's orbit. Alternatively, it could now already be in that dimming phase. In any case, any inner planets like Mercury the star might have (or had) are gone. And that is minor compared to what will happen when the star's core helium is used up (having fused to carbon and oxygen) and it becomes thousands of times brighter than the current Sun, as big as the orbit of Earth, loses its outer envelope, lights up a surrounding planetary nebula, and dies as a white dwarf. Phi Aquarii is sometimes listed as having a spectroscopic companion, though that is not confirmed. Like the Sun, the star will probably have to go through its long evolution by itself.
Written by Jim Kaler 11/05/10. Return to STARS.