PHI PER (Phi Persei). An amazing star, Phi Persei is an extreme system that inspires more than a bit of awe. Not part of any of Perseus's usual outlines, mid-fourth magnitude (4.07), Phi Per is just barely over the border with Andromeda and about midway between the classical figures of Perseus and Cassiopeia, so it's easy to miss, and that's too bad. In the Milky Way at a distance of 718 light years (give or take 31), Phi Per is dimmed by two-thirds of a magnitude (54 percent) by interstellar dust. Were the line of sight clear, it would shine at magnitude three (3.40) and might even have a proper name. It's a hot class B (B2) hydrogen-fusing dwarf, an emission-line star with a surrounding radiating disk in the mold of Gamma Cassiopeia and Zeta Tauri, indeed a "shell star" in which the disk is presented more or less edge on. Consistently, Phi Per is spinning madly, with an equatorial velocity of at least 430 kilometers per second, making it one of the fastest-rotating stars known. All "Be" stars are fast rotators, though just how the rotation factors into the creation of the surrounding disk is not very well understood. Like almost all Be stars, Phi Per is variable, though only slightly, wobbling between magnitudes 4.03 and 4.11. Of vastly more interest, Phi Persei is double with a strange hot companion that takes 126.7 days to orbit its brighter master. From the 25,400 Kelvin temperature found for Phi Per proper (the dwarf, Phi A), needed to estimate the high amount of ultraviolet radiation, and the distance, we get a luminosity of 16,100 times that of the Sun, which leads to a radius of 6.6 times solar and a rotation period of under 0.77 days. The disk's radius is roughly 10 times as large. Luminosity and temperature plus theory yield a mass of 12 Suns, which may be a bit high because of contamination of the visible dwarf's light by the odd companion, but not by much.

With a temperature estimated at 53,000 Kelvin, the companion is a "subdwarf O" star. Cool subdwarfs are low-metal stars of the Galaxy's halo. Very different, hot subdwarfs are stars that have lost their outer envelopes and are in some stage of development toward becoming white dwarfs. Phi Per B, now the lighter star, was once the mightier, which means it was slated to die first. In the process of evolution, it has lost most of its mass, a lot of it to Phi Per A, which is NOW the more massive, the two switching the dominant role. Algol, Beta Persei, shows much the same kind of action, though it's in an earlier stage. The disk around Phi A may then be the result of accretion from what is now the secondary (Phi B). Mass accretion toward the side of Phi A, the class B dwarf, then spun it up to its present state of ultra-rapid rotation. Phi A now seems to be shoveling some of its mass back to the subdwarf, Phi B, whence it creates at least an illusion of another disk, each star having one. Study with the Hubble Space Telescope suggests an orbital radius for the subdwarf of 1.1 Astronomical Units and respective masses for Phi A and B of 9.3 and 1.14 Suns, making the Be star (Phi A) too luminous for its mass (and too hot as well), no surprise given the intense activity and mass transfer going on. With such a high degree of mass loss and illuminating ultraviolet radiation, one might expect an extended planetary nebula (which has nothing to do with planets) to surround the system. Perhaps the evolution has not gotten that far. We don't know. The star, however, is a guide to a genuine and well-known planetary nebula, Messier 76, which lies just under a degree to the north-northwest. (Thanks to: D. R. Gies et al. in the Astrophysical Journal, 493: 440, 1998; S. Stefl, W. Hummel, and Th. Ravinius in Astronomy and Astrophysics, 358: 208, 2000; and W. Hummel and S. Stefl in Astronomy and Astrophysics, 368: 471, 2001.)

Written byJim Kaler 1/16/15. Return to STARS.