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ALMAAZ

(Epsilon Aurigae). Tucked to the southwest of Capella in Auriga is a prominent narrow triangle of stars. Older traditions call the southern two (Hoedus I and Hoedus II) "the Kids," though Almaaz (Bayer's Epsilon star), which shines at mid- third magnitude (2.99), is now commonly included. The Arabic name, in parallel with Capella (from Latin, "the she-goat"), means "the he-goat." One of the weirdest and least understood stars in the sky, Almaaz is a hot-end class F (F0, though some say A8) supergiant located a rather uncertain 2000 light years away. From a 7800 Kelvin surface a bit over an Astronomical Unit (Earth-Sun distance) in diameter (almost half again the size of Mercury's orbit), it radiates nearly 47,000 times as much energy as the Sun, its mass somewhere between 15 and 19 times solar. Surrounding it is a gaseous ring. Class F supergiants are relatively rare to start with, but that is only the beginning. Like so many stars, Almaaz is an eclipsing double star, but one of strange and magnificent proportions. Nothing about it is ordinary. The prototype eclipser, Algol (in Perseus), dims by about a magnitude every 2.87 days when a class K giant star passes in front of a small but bright hot class B star. Almaaz drops by about the same amount, roughly to the apparent brightness of Hoedus I (Zeta Aurigae). However in contrast to Algol, Almaaz's eclipses are 27.1 years apart and last for an amazing two years, testimony to the huge dimensions of the system. Odder yet, it is the big class F supergiant that gets eclipsed, by something that is vastly larger than it! And no one knows quite what. The prevailing model is that Almaaz is in mutual orbit with a star that is surrounded by a thick ring of obscuring dust set nearly edge on. Almaaz proper (the supergiant we see) and the mystery star are perhaps 30 Astronomical Units apart, the dust ring about the secondary star some 20 AU in diameter. The ring has some sort of doughnut hole in the middle, as Almaaz brightens a bit at mid-eclipse. We have little idea what lies in the dusty ring. One theoretical model says something of 4 solar masses, another of 15. It could be one star that has generated a disk through a fierce outflowing wind or (as more commonly believed) a pair of class B stars that are themselves in tight orbit. The last eclipse took place in 1982-84. The next will be in 2009-11, when you can watch and can bet that a great number of telescopes, both on the ground and in space, will also avidly be looking at it with a generation of instruments not yet built. Maybe then we can figure out what is going on. One thing almost certain is that Almaaz will someday explode.