Almaaz

Almaaz is a star that you may know as Epsilon Aurigae, part of a triangle of stars that is sometimes called the Kids. It is located southwest of Capella and its magnitude is just about 3. Capella is the Latin name for “she-goat” and Almaaz is Arabic for “he-goat”. It is an F class star though some classify it as an A8. It is located about 2000 light years away but that distance is uncertain. It’s diameter is just about 1 AU with a mass more than 15 times that of the sun. But the really strange thing about Almaaz is the way that it is eclipsed by it’s partner. While the prototypical eclipsing binary Algol goes through a cycle once every 2.87 days, Almaaz goes through a cycle once every 27.1 years. Remember how large this star is? Its eclipsing partner is larger! It seems unlikely that a star could be unseen and yet bigger than Almaaz so the assumption is that the eclipsing star includes a large dust cloud - perhaps a protoplanetary system. What is the evidence of that? First, the eclipse lasts so long that the most likely candidate is a star orbiting Almaaz at a distance of 30AU and the dust cloud is 20 AU in diameter. Second, the eclipsed star brightens a bit right in the middle of the eclipse - as if there was a hole in dust. The last eclipse of Almaaz started in 1982 and when you add 27 to that you get, hey, 2009. Indeed, the study of Almaaz is one of the highlights of the IYA 2009. The idea is that amateurs can help professional astronomers by taking astrometrical measurements before, during and after the eclipse. The eclipse should start in August and continue until early 2011.

As always with this irregular column, star information is easily found by entering the star’s name and “Kaler” in your favorite Internet search engine. In this case see http://www.astro.uiuc.edu/~kaler/sow/almaaz.html .