Al Anz — Stella doppia in Auriga

Al Anz (also known as Almaaz), Epsilon Aurigae, is one of the most mysterious stars in the sky — an F0 Ia yellow-white supergiant locked in an eclipsing binary with a huge, dust-shrouded companion. Every 27.1 years the system dims by about one magnitude for nearly two years as the companion's enormous dusty disk transits across the supergiant. The primary lies roughly 2,000 light-years away, shines intrinsically at some 37,000 solar luminosities, and has a radius perhaps 175 times that of the Sun.

Epsilon Aurigae is a naked-eye star just south of Capella, forming one of the "Kids" asterism inside the pentagon of Auriga. Normally magnitude 3.0, it fades to 3.8 during eclipses — the last full eclipse ran from August 2009 to May 2011, and the next will begin around 2036. Any small telescope reveals its warm creamy color. This is a terrific variable-star project: comparing it to nearby Eta Aur and Zeta Aur around eclipse times reveals a change any observer can follow by eye.

Al Anz, or "al-'Anz" (the she-goat), comes from the Arabic tradition that grouped this star with neighboring Zeta Aur and Eta Aur as the "kids" of the goat Capella. The star's long-period variability was discovered by Pastor Johann Heinrich Fritsch in 1821, but the eclipsing nature only became clear after Hans Ludendorff's detailed 1904 monograph. The nature of the companion's disk remained unsolved for over a century.