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.