ZETA PHE (Zeta Phoenicis). The farther south a star to northerners, and the farther north to southerners (meaning north or south of the equator), the less it is known to either. Anywhere other than the equator itself, there is always a hidden region of the sky. You have to be south of 34 degrees north latitude before Zeta Phoenicis (in the constellation of Phoenix of its own name, that of the Firebird) can be glimpsed. And pity for the large northern populations, because Zeta is a something of a southern counterpart of famed northern Algol -- or at least someday will be. An "eclipsing" binary (in which each orbiting star in turn gets in front of the other), this fourth magnitude (3.92) class B dwarf drops half a magnitude to nearly fifth (4.42) every 1.6697664 days (40 hours, 4 minutes, 28 seconds), with a smaller eclipse of 0.18 magnitudes centered right in the middle. The deeper eclipse is well-visible to the naked eye. The star is also a "double- lined spectroscopic binary," meaning that we can get velocity information from the two stars and obtain a wonderful handle on both the nature of the system and on the individual components. The pair consists of orbiting class B (B6 and B8) hydrogen-fusing dwarfs with respective masses 3.93 and 2.55 times that of the Sun, surface temperatures of 14,100 and 11,800 Kelvin, radii 2.85 and 1.85 times solar, and luminosities 290 and 60 times solar. The temperatures and luminosities fitted to the theory of stellar evolution give satisfyingly similar masses. Each of the two eclipses lasts for just over 5 hours from start to finish. The "primary" (deeper) of the two eclipses occurs when the smaller star is in front of the bigger and hotter one, blocking some 60 percent of its light. The orbital plane is tilted almost into the line of sight, tipped only by a few degrees, so the eclipse is close to central, which leaves only a ring of light surrounding the bigger star. The lesser in-between secondary eclipse takes place when the larger star completely hides the smaller, leading to a "flat-bottomed" total eclipse that lasts a bit over an hour. The separation between the two is a mere 0.05 Astronomical Units, only four times that of the larger, brighter, B6 star. Though the orbits are eccentric by only about one percent, tides raised in each star by the other cause the orbital major axis to rotate over a 32.5 year period, which allows a test of the theory of the stars' internal constructions. The stars' rotation periods are somewhat shorter than the orbital period, and therefore not synchronized with the orbital revolution. Zeta Phe is called a "detached binary," as the two stars do not exchange any matter (that is, there is no "contact" between the two). Someday, however, when its internal fuel runs out, the more massive primary star will become an expanding giant and will begin to lose its matter tidally to the smaller star. The more massive will become the less massive, and another "real" Algol will grace the sky. Set off by at least 600 AU from the pair is a much dimmer 7th magnitude star that, from its brightness, must be a class F dwarf whose orbital period would be over 5000 years, and from which the inner pair would appear (to the naked eye) as a mere point.
Written by Jim Kaler. Return to STARS.