PI AQL (Pi Aquilae). "A close double star on the Eagle's northern wing, about 3 deg N. 1/2 W. from its lucida, A 6, pale white; B 7, greenish. This beautiful object may be termed a miniature of Castor" (Smythe and Chambers, speaking from 1881). The "lucida" is first magnitude Altair, the brightest star in Aquila, the Eagle, the numbers rounded-off magnitudes, which was about all astronomers could do with the visual estimates of the 1800s. The "greenish" color of the fainter, Pi Aquilae B, is an illusory contrast effect. The binary nature of Pi Aql, 514 light years away (give or take 42), was discovered by (who else?) William Herschel, who in 1783 also made the first measure of angular separation of 1.4 seconds of arc, the same as found in 2015, so the stars are keeping good pace with each other and are obviously gravitationally joined, though no significant orbital motion has yet been detected. There has been considerable confusion regarding the spectral classes and about which might be ascendant. The primary now seems to be a class F (F9) giant (once thought to be an F2 dwarf, even a G8 giant), while the secondary is most likely a class A3 dwarf and white, not "greenish." The problem is that the stars are so close together that the light of each contaminates that of the other, which makes evaluation of magnitudes difficult as well. We adopt visual magnitudes of 6.34 and 6.75, which together give a combined magnitude of 5.77, very close to that listed by others. A third suggested component, thirteenth magnitude Pi Aql C, 36 seconds of arc away, has shifted 4.4 seconds over the past 125 years, and can be dismissed as an optical alignment. There are no accurate temperature measures, so we adopt respective temperatures of 5900 and 8700 for Pi A and B, which with distance yields luminosities of 61 and 40 times that of the Sun and radii of 7.5 and 2.8 solar radii. Theory gives masses of 2.6 and 2.3 Suns, an age of some 600 million years, and shows that while the slightly lesser star is indeed a dwarf, the primary is more of a subgiant in cooling transit between dwarf and true gianthood, before it begins to make its first ascent brightening in preparation for the firing up of its now-dead helium core, thus explaining the similarity of properties of the two stars. At birth, they would have been seen as a sparkling pair of class B9 stars, one slightly brighter than the other. Given their angular separation and distance, they must be at least 220 Astronomical Units apart (the foreshortening not known) and take more than 1500 years to circuit one another. Given their minimum physical separation, from each the other would shine with the light of 500 full Moons. (If the primary is as cool as G8, 4900 K, then because of the addition of infrared radiation, its luminosity goes up to 76 Suns, the radius to 12 solar, and the mass to 3 Suns, theory then showing the star to be an actual helium-fusing giant, revealing how much there is to learn about the system.)
Written byJim Kaler 10/07/16. Return to STARS.