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AL KAB

(Iota Aurigae). Some bright stars do not get the respect they deserve, in part because they have no well-known names, because they carry middling Greek letter names, or are overwhelmed by others in the constellations of great fame. So it is with Auriga's Al Kab, short for Al Kab dhi'l Inan, from Arabic meaning "the shoulder of the rein holder." The name was originally applied to Gamma Aurigae, which actually is more a part of Taurus, and is now always referred to as Elnath (Beta Tauri). Perhaps as a result, Al Kab also carries the more modern moniker "Hassaleh." When we look at Auriga, we more admire brilliant Capella, and gaze at the triangle made by "the Kids," certainly not at Al Kab. Though at bright third magnitude (2.69, but slightly variable) and fourth brightest star in the constellation (third if you exclude Elnath), Bayer still gave it the Greek letter "iota," preferring lower letters for the Kids: Almaaz, Haedus I, and Haedus II (Epsilon, Zeta, and Eta). At first glance, Al Kab is "just one more orange class K (K3) giant." But one with a difference. First, it is a "bright giant," more luminous than most of the class. From its rather large distance of 510 light years, it shines with a radiance 3100 times that of the Sun from a surface with a well-determined temperature of 4390 Kelvin. With a diameter 101 times that of the Sun (half the orbital size of Earth), the star is so big that the angular diameter of its disk can be measured. Eight times the solar mass, the dying orange giant, which is most likely fusing helium to carbon in its deep core, approaches the limit at which stars explode -- but is still clearly under it, and will produce no more than a massive white dwarf. Beginning life as a hot class B star similar to Graffias-1, it has taken some 35 million years since birth to reach its present state. Al Kab is also among the sky's brightest X-ray producing "hybrid stars." Lesser giants produce X-rays from hot coronae similar to the one found around the Sun, while among more advanced giants, the coronae are replaced by cool, outward-flowing winds. In Al Kab, we see both, which certainly elevates the star to the status achieved by its constellation-mates. Thanks to Jeff Bryan, who suggested this star.