PI CAP (Pi Capricorni). Enter here with brave caution unless you
like uncertainty and confusion. Fifth magnitude (5.25) Pi
Capricorni, in western Capricornus (the
unlikely Water Goat), a few degrees south-southeast of (and pointed
to by) brighter Alpha and Beta Cap, might better be known as the "if"
star, as so much has to be assumed: "if this, than that," etc. Hardly
anybody has observed it, the star getting a miserable 42 citations
over the past century and most of these useless. We do know at least
that it's a triple star. The luminary, Pi Cap A is a blue-white class
B star, the Bright Star Catalog giving it as a B8 giant or even bright giant,
whereas later classification makes it to be a much hotter B4 dwarf, quite a divergence. The
color favors the former. Right next to it, a tenth of a second of
arc away, is an eighth magnitude (7.9) companion labelled Ab, while
fainter Pi Capricorni B at magnitude 8.5 lurks farther away, currently
3.2 seconds. It's kept pretty good track with Pi A, and is almost
certainly gravitationally bound to it. (Fourteenth magnitude Pi C
at 38 seconds of arc distance, is speeding along far too fast, and
just lies in the line of sight.) The companions could well have
obscured the class of "A," making it appear too cool. Who knows?
Given some uncertainty in the magnitude of Pi A, we adopt a simple
5.2. Rather amazingly, there are no temperature measures. If Pi-A
is indeed a B8 giant, then its surface should radiate at a temperature
of 12,000 Kelvin. From their classes (A4 and A8 dwarfs) Ab and B
should be at 8500 and 7700 K. From a rather uncertain (that word
again) of distance of 545 light years (give or take 53) and allowing
for some ultraviolet
radiation from Pi-A, the three should have luminosities of 315, 15,
and 8.6 times that of the Sun, radii of 4.1,
1.8, and 1.7 solar radii, and from theory masses of 3.8, 1.9, and
1.7 Suns. At a minimum distance of 17 Astronomical Units from Pi-Aa,
Pi Ab takes at least 29 years to orbit, whereas at least 535 AU away,
Pi-B must take more than 4600 years to make its rounds.
But then there is the matter of Pi-A. If it's a B4 dwarf, it's much
hotter, 17,000 or so Kelvin. The increased ultraviolet contribution
gives it a luminosity of 640 Suns, almost double that figured before.
Theory in turn suggests a mass of 5.3 Suns. The increased mass
shortens the minimum orbital periods of the two class A companions
to 24 and 4200 years, not that much of a difference. After stellar
evolution has taken its turn, and the stars slough off their outer
envelopes, Pi Aa, Ab and B will turn into white dwarfs with masses of
0.8 (or if the hotter choice is made above, 0.9), 0.62, and 0.60
solar masses, providing the inner two do not interact during their
giant stages.
Written byJim Kaler11/07/14. Return to STARS.