PHI AND (Phi Andromedae). At the eastern end of the northern string of stars that makes Andromeda, fourth magnitude (4.25) Phi Andromedae shines modestly from a large distance of 735 light years, showing that it is luminous indeed. Not once, but twice, as the star is a tight double with a separation of only half or so of a second of arc, making it a very difficult target to resolve visually. Taken as one, the star appears as a class B (B7) dwarf.
Phi And Phi Andromedae sits in the middle of this seven-degree-wide portion of northeastern Andromeda. The four brighter stars to the left are, from top to bottom, 51, 49, Omega, and Xi Andromedae, (Xi the one on the right). The trio of stars to the right of Phi are in Cassiopeia, the top one Omicron Cas, the lower one Pi. A higher resolution view reaches to mid-tenth magnitude.
Separately, the brighter, Phi And A, seems to be a fifth magnitude (4.54) B6 subgiant, while the fainter, Phi And B, is a sixth magnitude (5.55) B9 dwarf (the magnitudes uncertain and scaled to fit to the measure of the combined stars). There are no temperature measurements as the stars are too close for ready observation. Assuming 14,000 for B6 Phi And A, correcting for 0.2 magnitudes of interstellar dust extinction as well as for a lot of ultraviolet light, gives a luminosity of 1980 times that of the Sun, a radius 7.6 solar, a mass of about 5 1/2 solar, and confirmation that the star is indeed a subgiant and is about to give up core hydrogen fusion. An assumed temperature for Phi And B, with the same kinds of corrections, gives 350 solar luminosities, 5.1 solar radii, 3.7 solar masses, and confirmation of dwarfhood. As best we can tell, given the difficulty of measure, the pair orbit each other with a fairly long period of 372 years at an average separation of 101 Astronomical Units, with an eccentricity that takes them between 140 and 80 AU. They were closest together (periastron) in 1886 and will be farthest apart (apastron) in 2075 (a long wait). Application of Kepler's laws yields a total mass of 7.5 solar, short of the sum of 9.2 obtained from stellar structure theory. Given that only a partial orbit is available (we have yet to see them make a full orbit), the orbital parameters surely contain errors, so the difference is not surprising. In fact it is satisfying that the two determinations come that close. Phi And A -- like Gamma Cas and Zeta Tauri -- is a "B- emission" ("Be") type star, one with a circulating circumstellar disk that radiates emission lines (the origins of which are not at all clear). Be stars are fast rotators, in the range of 200-300 kilometers per second (which clearly has something to do with the phenomenon). Phi And A's observed rotation speed is relatively low, however, only 81 kilometers per second, implying that the rotation axis is rather well tilted toward us (such speeds being projections against the sky and therefore lower limits). Such a tilt is consistent with the nature of the disk, which does not seem to superimpose itself against the star (that is, Phi And A is not a "shell star" like Alpha Arae, Eta Centauri, and several others that dot the sky).
Written by Jim Kaler. Return to STARS.