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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 "Erakis (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 (25,600 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.