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.