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MARKAB

(Alpha Pegasi). Markab epitomizes what seems almost to be a celestial joke, a comic opera of sorts in which stars scramble their names and appear as different characters on the sky's stage. Four stars make the Great Square of Pegasus, Markab (Alpha) at the southwestern corner, Scheat (Beta) at the northwestern, Algenib (Gamma) and the southeastern, and Alpheratz (Alpha Andromedae) at the northeastern, this last star linking the Winged Horse to Andromeda. "Markab" comes from an Arabic phrase meaning "the horse's shoulder," but in more recent times was mistakenly taken from what is now Scheat. But Scheat's name ("the shin" was mistakenly taken from "Skat," the Delta star in Aquarius. Continuing the confusion, Algenib's name was taken from Mirphak (the Alpha Star in Perseus), whose alternative name is ALSO Algenib. Not to be outdone, "Alpheratz" may have come from an original name for Scheat. The Greek letter system is not much better. As a linking star, Alpheratz -- Alpha Andromedae -- is also Delta Pegasi. Moreover, Mirfak, the Alpha star, just barely second magnitude (2.49), is only third brightest in the constellation, and is exceeded by both the Beta star (Scheat), by of all things, Epsilon (Enif), as well as by the Delta star (Alpheratz of Andromeda). There will be a quiz on all this next Tuesday. Markab itself is much clearer. It is a relatively hot (class B, though at the cool end) giant, appearing not much different from (though visually fainter than) Regulus in Leo. Its distance of 140 light years leads to a total luminosity 205 times that of the Sun (including some invisible ultraviolet), which with the star's temperature (10,500 Kelvin) yields a radius 4.3 times solar. Just a hair over three solar masses, and still spinning fairly rapidly (its rotation period under 1.5 days), Markab has just begun to die. If hydrogen fusion has not already ceased in its core, it is very close. The star is in a sense clinging to its lifeline on the hydrogen-fusing normal "main sequence" of stars and is about to leap into the abyss, in which it will quickly expand, slow its rotation, and become a much cooler orange giant (perhaps someday looking like Kornephoros in Hercules). It will then brighten to many times its current luminosity to die finally as a massive white dwarf like Sirius-B. Some stars are remarkable for their chemical compositions, others for their multiplicity, yet others for strange variability. Though there is some evidence from Markab's motion for a small stellar companion, the star is famed for none of these. It is in fact noted for its very normality, behaving quite like it should. Such normalcy is extremely important in stellar astronomy, as it provides a bedrock standard against which to measure the odder members of the stellar zoo. Of the 100 publications in which Markab has been mentioned in the past 15 years, nearly all use it as such a base. Go out and enjoy a perfectly normal, and quite admirable, star.