ALFIRK (Beta Cephei). In mythology, Cepheus (the King) pales beside Cassiopeia (the Queen), who is central to the story of
Andromeda. And so does his
constellation, which is among the fainter of those of the Perseus
myth. Yet dim Cepheus is not without his own glory. As the most
northerly of the Perseus set, most of Cepheus is circumpolar from
as far south as 30 degrees north latitude. In addition to
Herschel's "Garnet Star," it also
contains variable stars that are prototypes of their classes.
Foremost is Delta Cephei, which gave the name "Cepheid variables"
to the world (the set including Mekbuda
in Gemini and even Polaris), regularly oscillating stars whose
luminosities are proportional to their few-day periods and that
provide us with the best distance indicators to nearby galaxies.
The other (far less- known) class is epitomized by Alfirk. The
name, from an Arabic phrase for both Alderamin (the Alpha star) and Alfirk,
simply refers to "the two stars," but might also might mean a
"flock of sheep." Here Bayer got it right, as the Alpha, Beta, and
Gamma stars are in actual order of brightness. Though toward the
faint end of third magnitude (3.23), Alfirk is in its own right
quite the magnificent star. A hot (26,700 Kelvin) blue class B
(B1) star, it is faint only because it is fairly far away, shining
with a luminosity 14,600 times that of the Sun from a distance of 600 light years. While
not actually involved in nebulosity, Alfirk is just hotter than the
dividing line at which stars can ionize and illuminate any
surrounding gaseous interstellar matter. Alfirk's real claim to
fame, however, is that it is the principal of the "Beta Cephei
stars," which subtly vary by a few hundredths of a magnitude with
multiple periods and which include Mirzam
(coincidentally the Beta star in Canis Major). Alfirk's chief
period is only 4.57 hours, during which it varies from magnitude
3.16 to 3.27 and back. Like all Beta Cephei stars, however, Alfirk
varies -- pulsates -- with many periods at one time, much smaller
subtle changes taking place with periods of 4.72, 4.46, 4.43, 4.88,
and 4.30 hours in addition to 6 and 12-day rotational modulations.
Alfirk is classed as a subgiant (even a giant), as a star at or
near the end of its hydrogen-fusing lifetime. Many such stars
pulsate in similar fashion, their lost sense of perfect stability
coming from the valving of the flow of heat far below their
surfaces (rather like the mechanism that drives the much cooler
Cepheids). In addition, Alfirk is a "B-emission" star that expels
matter from its surface and has a magnetic field about 100 times
greater than does the Earth. Two smaller and dimmer class A stars
accompany it. The inner one is only about 45 astronomical units
away, and takes around 90 years to orbit. Much farther out is a
visual companion easily seen in a small telescope. At least 2400
AU away, it must take at least 30,000 years to make a circuit.
With a mass a dozen times that of the Sun, Alfirk is probably too
great a star to make a white dwarf, and seems to be just above the
limit at which stars explode, though errors in measurement make us
uncertain about the final fate.