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 heel 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. It is a "bright giant," more luminous than most of the class. Its placement in the Taurus-Auriga part of the Milky Way and a rather large distance of 510 light years also causes it to be dimmed by roughly 0.6 magnitudes as a result of interstellar dust absorption. Were the dust not present, Al Kab would be a mid-second magnitude star. When the dust absorption and infrared radiation from a 4390 Kelvin surface is taken into account, the star is see to shine with a total radiance 5400 times that of the Sun. With a then-calculated diameter 128 times that of the Sun (60 percent the orbital size of Earth), the star is so big that the angular diameter of its disk can be measured (which gives a somewhat smaller radius of 106 solar). Seven or nine times the solar mass (depending on just how the star is evolving), the dying orange giant is most likely fusing helium to carbon in its deep core. It is also near the limit at which stars explode as supernovae. If it does not, it will die as a massive white dwarf. Beginning life a hot class B star similar to Graffias-1, it has taken between 30 and 45 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.
Written by Jim Kaler. Return to STARS.