The curtain is rising on a one-act drama in the night sky that was last performed in the early 1980s and won't come round again until 2036. Fortunately, backyard stargazers will not only have the best seats in the house — they've also been invited on stage to participate!

How to find Epsilon Aurigae

Epsilon Aurigae is easy to find near the bright star Capella. It's normally magnitude 3.0 — but later this year it will dim to 3.8. This change in brightness should be evident when compared with the magnitudes of nearby stars.

Sky & Telescope diagram

Our star: Located less than 4° from brilliant Capella, Epsilon Aurigae is a seemingly normal F-type supergiant with about 300 times the Sun's diameter and 15 times its mass. It's about 2,000 light-years away. Right now Auriga rises in the northeast about 9 p.m. and is high enough for quality viewing by midnight.

The plot: In 1821 astronomers discovered that Epsilon had unexpectedly dimmed to about half its normal brightness, from magnitude 3.0 to 3.8. This mysterious dimming occurred again in 1847, 1874, and 1903, by which time astronomers had figured out that the star was part of an eclipsing binary system.

The plot thickens: Spectroscopic observations revealed that the eclipse-causing companion must be nearly as massive as Epsilon — yet astronomers have yet to detect any light from it. As best they can tell, the dimmings are due to an oblong, opaque disk that's cloaking the companion star (or stars).

Your role: The latest eclipse from this is just getting under way, and Epsilon Aurigae should gradually dim until early winter in the Northern Hemisphere. (Sorry, its declination makes for poor viewing for southern observers.) It will remain faint throughout 2010 before slowly brightening to its normal luster by mid-2011.

Modern observatories aren't equipped to study stars this bright — but many backyard astronomers are. So the American Association of Variable Star Observers has just launched a program that allows amateurs of all stripes to help solve this centuries-old enigma.

Astronomers want to know what, exactly, is passing in front of the star Epsilon Aurigae every 27 years.

S&T: Casey Reed

Unlike other "citizen science" efforts, the AAVSO's Citizen Sky lets you experience all aspects of scientific research. Your visual or electronic estimates of Epsilon Aurigae's brightness are of course welcome — that's been the bread and butter of the AAVSO's work since its founding in 1911. But you can also use the online tutorials to learn how to analyze data, create and test your own hypotheses, and write up findings for publication in astronomy journals. You can work alone, join a team of observers, or form your own.

Funded by the National Science Foundation, Citizen Sky is a collaboration of the AAVSO, Denver University, Chicago's Adler Planetarium and Astronomy Museum, Johns Hopkins University in Maryland, and California Academy of Sciences. Its lead astronomer, Robert "Dr. Bob" Stencel (University of Denver), studied Epsilon Aurigae extensively during its previous eclipse.

"Our goal is to introduce the public to authentic science and at the same time use this talent to help astronomers," notes AAVSO director Arne Henden.

As it turns out, I've just started teaching observational astronomy to a small class of high-school students, and "Citizen Sky" will be a perfect project for them. I know they'll enjoy getting a taste of real-world science — I'm sure you will too.


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