ALFIRK (Beta Cephei). In mythology, Cepheus (the King) pales beside Cassiopeia (the Queen), who is central to the story of
Andromeda and Perseus. And so does his constellation, which is the faintest of those
of the myth. Yet dim Cepheus is not without glory. As the most
northerly of the Andromeda group, most of Cepheus is circumpolar from as far south as 30
degrees north latitude. In addition to Herschel's "Garnet Star" and the magnificent eclipsing
binary VV Cep, it also contains two
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, Eta
Aquilae, 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, lesser-
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 did the expected, as the
Alpha, Beta, Gamma, and Delta stars are
close to being in actual order of brightness (Beta and Gamma nearly
the same), Alfirk toward the faint end of third magnitude (3.23).
A hot (26,900 Kelvin) blue class B (B1) "subgiant" (but see below),
it's faint only because it is fairly far away, shining with a
luminosity 23,000 times that of the Sun from
a distance of 685 light years (second Hipparcos reduction) give or
take 43. While the star is classed as a subgiant (even a giant), its luminosity and
temperature show it really to be a 13.5 solar mass dwarf that is still approaching
the end of its 11 million year hydrogen-fusing life.
Of most significance, Alfirk is the prototype of the "Beta Cephei
stars," which subtly vary by a few hundredths of a magnitude with
multiple periods and which include brighter Mirzam (Beta Canis Majoris, which is
sometimes considered to be the prototype). 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. 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).
Alfirk had an odd reputation as a "B-emission" star with a
radiating circumstellar disk, the best known of which are Gamma Cassiopeiae and Zeta Tauri. The "Be" phenomenon is related
in an unknown way to fast rotation. Yet Alfirk is spinning slowly
with a speed of just 33 or so kilometers per second, far below that
of the usual Be star. The Be star here, however, seems not to be
Alfirk proper, but a dimmer B5 dwarf companion of four or so solar
masses 0.25 seconds of arc (50 Astronomical Units) away that is
spinning at 230 km/s and takes around 90 years to orbit. Moreover
one of them, perhaps the companion, is magnetic with a field some
100 times greater than Earth's. Much farther out is a ninth
magnitude visual companion, an A dwarf easily seen in a small
telescope. At least 3000 AU away from Alfirk, the neighbor must
take at least 40,000 years to make a full circuit. Alfirk proper
is too massive to make a white dwarf and seems
destined to explode as a supernova, the event probably
setting the companions free to roam the Galaxy all alone.
Written by Jim Kaler 10/27/00; revised
5/21/13. Return to STARS.