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.