31 AQL (31 Aquilae). Some stars are singled out for their brightnesses, some for their positions within a constellation or on the sky, others for their duplicity or even multiplicity, more for strange characteristics, yet more for their sunlike natures. Here, fifth magnitude (5.16) and relatively faint, is one that firmly belongs in the last two categories. 31 Aquilae, in Aquila to the northwest of first magnitude Altair, is a nearby (just 39.5 light years away) class G8 subgiant, whose 5570 Kelvin temperature is just a bit cooler than that of our 5780 Kelvin class G2 Sun. That and a luminosity 1.75 times that of the Sun leads to a radius of 1.4 solar and a slightly subsolar mass of 0.95 times that of the Sun. On the basis of lower mass alone, the star should be LESS luminous than our Sun, not more. The difference is in 31 Aql's age, the star telling us quite vividly what is going to happen to US. As dwarf stars get older and fuse their internal hydrogen to helium, they slowly brighten and swell, the result of the contraction and heating of the deeply buried nuclear-fusing core. 31 Aquilae is thus an old star, probably more than 90 percent of the way through its 11 billion year dwarf lifetime, hence its status as a "subgiant." Our much younger 5-billion-year-old Sun, which is halfway through its normal lifetime (before it too becomes a subgiant then giant) has not yet had the time to brighten all that much from its birth-luminosity. But 31 has much more going for it than just being a sort-of solar clone. It's also one of the sky's odd "super-metal-rich" stars, whose iron content (relative to hydrogen) is remarkably high, in this case double that found in the Sun (and falling in league with a few others like Xi Puppis and Alpha Indi). Other elements -- silicon, magnesium, sulphur -- follow iron's lead; carbon and oxygen are up as well. 31 is clearly "not from these parts," as the metal content of our part of the Galaxy is more or less (even under) solar. Attesting to that is a very high velocity relative to the Sun of 122 kilometers per second, some eight times normal, showing the star to be a visitor from a different Galactic location, likely the high-metal central bulge. High metal stars have a propensity for planets, though so far as we know, 31 Aql has none. Even though old, the star still shows some an erratic magnetic behavior that, unlike the 11-year solar cycle, reveals no periodic behavior. While seemingly accompanied by three "neighbors" of ninth and tenth magnitude between 80 and 140 seconds of arc away, the so-called companions are just line-of- sight coincidences, our wandering, older, sunlike, metal-rich star apparently all alone.
Written by Jim Kaler 9/04/09. Return to STARS.