PI PSA (Pi Piscis Austrini). Some stars are so confused and
confusing that you almost feel sorry for them. You look on a
detailed star map and note with some excitement that Pi PsA, a
fifth magnitude (5.11) class F dwarf (given as F0+F3) in Piscis Austrinus (the Southern Fish) well to
the south of first magnitude Fomalhaut, is marked as a Cepheid variable star, a
classic form of variable in the mode of Delta Cephei, Zeta
Geminorum, and Eta Aquilae. Deeply
important, because their absolute brightnesses are tied to their
variation periods, Cepheids are our keys to finding the distances
of galaxies and to defining the expansion properties of the
Universe. Then you watch the star, and it does not DO anything,
just sits there staring back at you. And then you realize that,
wait a minute, the thing is a class F DWARF, and Cepheids are class
F and G supergiants and bright giants. The designation as Cepheid,
made in the mid-twentieth century and placed into books and
catalogues, is hopelessly wrong, as further studies showed no
variation at all to within a couple percent. Not only is Pi PsA
not a Cepheid, it is not any kind of variable at all! Then how
about the class? The double-designation F0+F3 implies a pair of bright stars. And
indeed, Pi PsA is a spectroscopic
binary with a well-determined period of 178.3177 days. So
assume two stars, and from their classes get their temperatures of
7400 Kelvin for Pi A (close to the measured presumed-single-star
value of 7280 K) and 6900 for Pi B, as well as their estimated
brightness ratio, and you get luminosities of 3.4 and 2.6 times
that of the Sun. Which are both impossibly
too low for the temperatures. Then realize that orbital studies
show the spectrum of just ONE star, not TWO! Pi PsA B thus must be
a dim bulb that is not recorded, and the double class is wrong.
From the measured temperature, the class should be around F1, and
the luminosity (now assuming a single star) should be 6.0 solar,
and all falls into place for a 1.6 solar mass young dwarf about a
third of so of the way through its 2.2 billion year lifetime. With
a radius of 1.5 Suns, it takes less than roughly 3 days to rotate
(from a not unsurprisingly very poorly-determined rotation speed).
Assuming the companion, which is real, has insignificant mass, from
Kepler's Laws it orbits at a distance of 0.9 Astronomical
Units, an actual measured eccentricity taking it between 1.3 and
0.4 AU from its more massive mate. About all that is left to be
said, which is reasonably certain, is that the star is as bit metal
poor, with half the iron found in the Sun. At the end, while one
hopes there are not too many naked-eye stars as fraught with
problems as this one, we do see that there is still much work to be
done among them.
Written by Jim Kaler 12/25/09. Return to STARS.