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.