KAPPA AQL (Kappa Aquilae) with a look at neighboring U AQL (U
Aquilae). Massive stars attract us, and unassuming Kappa Aquilae
does not disappoint. The southernmost Greek-lettered star in Aquila (the Eagle), just five degrees
north of the border with Sagittarius, Kappa
Aql is a hot (26,400 Kelvin) class B (B0.5) "giant" (but see below) 1700 light
years away, give or take a couple hundred. With such a distance
within the Milky Way, the star is dimmed by
as much as 0.93 magnitudes by interstellar dust. With a clear
line of sight, Kappa would brighten to magnitude 4.02. The amount
of dimming is contended, however, another study giving 0.69
magnitudes. We'll go with the heavier obscuration. Factoring in
a lot of ultraviolet
radiation, Kappa Aql shines with the light of 55,500 Suns, which gives a radius of 11.3 solar. As
are most stars of its class, Kappa Aql is a very fast rotator, spinning
at the equator by at least 267 kilometers per second. Yet there seems
to be no circumstellar disk that would make it into a "B-emission"
(Be) star like Gamma Cas or Zeta Tauri. Theory suggests a mass of 17
solar masses if the star is a dwarf near the end of its
hydrogen-fusing lifetime of 10 million years, or slightly lower, 15
Suns, if it has already run out of hydrogen fuel in its core and has
become a subgiant (such
stars commonly spectrally classed as "giants"). The lesser dimming
gives a luminosity of 44,000 Suns but pretty much the same masses.
With a mass clearly above the theoretical limit of 8 to 10 solar masses
for eventual explosion, Kappa Aql does not have very long to live
before it blows up as a grand supernova. The star may be
unassuming now, but if it were to go off tonight, it would shine at
magnitude -8, roughly 15 times the brightness of Venus. So keep your eye on the Eagle: you
never know.
While Kappa Aql is a single star, it does provide a guide to another
of some interest. A pair of degrees due west lies the supergiant (or bright giant)
Cepheid variable U Aquilae. Cepheids are highly
evolved, pulsating bright giants and supergiants that typically vary
by a magnitude or so over a period of a few days. U Aql changes from
magnitude 6.08 to 6.86 and back every 7.024 days, while the class
goes from F7 to G1. Classical Cepheids such as Delta Cephei, Eta
Aquilae, Zeta Geminorum, and our "U"
are crucial to the calibration of the period-luminosity relation,
from which the distances of other galaxies (and the nature of the
Universe) can be derived.
U Aql, however, is not very good at it. Direct parallax gives a distance of
899 light years, while the period-luminosity relation (including 1.3
magnitudes of dimming by interstellar dust) gives some 2000. The
parallax is probably the culprit, but nobody knows. If at the
distance given by the pulsation period, U Aquilae, with a mean
temperature of about 5700 Kelvin, would shine with the light of 2800
Suns and carry a mass of around six Suns. U Aql, however, is not
single. A 7.6 magnitude companion hovers at a close but
unknown separation. Subtraction of its light increases the
period-luminosity distance to 2500 light years and makes the
discrepancy with parallax even bigger. More obvious is 11th
magnitude U Cephei B 1.6 seconds away. From its brightness, it's
probably a class A dwarf. Given a mass of two Suns, it's at least
122 Astronomical Units away from U proper and must take at least
14,000 years to make a full orbit. Half a minute of arc away is 14th
magnitude U Aquilae C, which is just an aligned coincidence, gladly
removing it from the already-complex analysis of the star.
Written byJim Kaler 9/25/15. Return to STARS.