PI AND (Pi Andromedae). The focus in Andromeda is usually on the great Andromeda Nebula, M 31, and if not that, on Andromeda's jewel of a double star, Almach, Gamma Andromedae. No one pays that much attention to the lesser stars that make the figure, let alone seemingly obscure Pi Andromedae, which glows softly at fourth magnitude (4.36) just to the north of third magnitude Delta And. A closer look reveals a bit of a surprise, of a hot (15,000 Kelvin) blue- white class B (B5) dwarf, one that is not just double, but quadruple and possibly quintuple. With a luminosity 2000 times that of the Sun (after allowance for a lot of ultraviolet light and 0.1 magnitudes of interstellar dust absorption), the star is clearly much too bright for its class. Which is no surprise, since the spectrograph reveals that Pi And proper consists of two nearly identical stars in close orbit. Assuming that they are truly closely similar (which they may not be), each shines with the light of 1000 Suns, which leads to radii of 4.7 times solar, masses of 5 times that of the Sun, and an age for the system of 80 million years, which gives them 15 or so million years left to their hydrogen-fusing life. (One study, however, suggests masses of 5.8 and 4.8 solar, which is not that far from equality.) A slowish equatorial rotation of but 30 kilometers per second gives a rotation period of under 8 days (but for which star is unclear). The pair orbit each other every 143.53 days. Interferometric observations reveal an average separation of 1.3 AU, a fairly high eccentricity taking them from as close as 0.6 AU out to 2.1 AU.
Pi And The two class B stars that make Pi Andromedae elliptically orbit nearly in the line of sight every 143.53 days at an average separation of just 1.3 Astronomical Units. (The brighter of the nearly identical pair is placed at the orbital focus; in reality they orbit each other around a common point close to midway between them.) Note the small scale: the tick marks on the axes are just 0.005 seconds of arc apart, making the pair impossible to separate visually through the telescope: the orbit is instead studied spectroscopically and via the interferometer. (From an article by C. A. Hummel et al. in the Astronomical Journal, vol. 110, p. 376, 1995 as given in the Sixth Catalog of Orbits of Visual Binary Stars, W. I. Hartkopf and B. D. Mason, U. S, Naval Observatory.)
The orbital plane is tilted nearly into the line of sight (which means that the stellar rotation poles may be presented to us rather perpendicularly, giving credence to the rotation period). Kepler's laws, however, give a combined mass of 15.7 solar, nearly 60 percent greater than that given by evolutionary theory, which suggests that the measured orbital size may be some 15 percent too high (not surprising, given the difficulty of interferometric measurement). At a much greater distance of 36 seconds of arc resides the eighth magnitude class A5 dwarf Pi And B, making the star a visual binary as well. With a separation of at least 7200 AU, the Pi And B would take at least 175,000 years to orbit the inner close binary. From Pi And B, the inner binary would shine with the total light of 8 full Moons, the stars maximally separated by around half a minute of arc. Farther out, 55 seconds of arc away from the close binary. is 11th magnitude Pi And C, whose membership is doubted. If it's real, then it is a mid-K dwarf at least 11,000 AU away that takes at least 350,000 years to make a circuit. Then a surprise, as an examination of the region close to the inner binary revealed the possibility of yet another member, a class K dwarf perhaps 40 AU out that would take some 80 years to orbit. Things are, as usual, not what they seem to be with the naked eye, or for that matter, even with the telescope.
Written by Jim Kaler 12/19/08. Return to STARS.