This originally appeared in our October 2009 issue.

A celestial sleight-of-hand is underway. The bright star Epsilon (ε) Aurigae began to dim this summer. Bit by bit, it will darken by almost a full magnitude on the astronomical brightness scale, becoming 2.5 times dimmer. Then it will languish at this shadowed state for the next year or more before slowly returning to its normal brilliance in the constellation Auriga the Charioteer.

Astronomers strongly suspect the dimming is some sort of eclipse event, in which an orbiting companion periodically passes in front of Epsilon Aurigae and blocks a portion of its light. Since 1821, astronomers ahve seen the star fade 7 times, every 27 years or so. And yet, as predictable as these eclipses have been, the underlying cause remains one of the oldest mysteries of modern astronomy.

The physical nature of the object that eclipses Epsilon Aurigae remains unknown. To produce such long eclipses, the obscuring object must be incredibly large — large enough that if it were placed at the center of our solar system, its surface would reach almost to the orbit of Uranus.

Further complicating matters, the eclipsing body does not appear to obscure Epsilon Aurigae entirely. Throughout the eclipse, a portion of Epsilon Aurigae’s light remains visible, as if the object obscuring it either doesn’t cover it entirely or covers it like the lenses in a pair of sunglasses — evenly blocking out a certain percentage of light at every wavelength but letting the rest through.

With each eclipse, a new generation of astro-nomers has attacked this mystery, coming at it each time with a new set of instruments. Yet, each time the new knowledge has only deepened the mystery. As astronomer Otto Struve wrote in 1962, “The history of the eclipsing binary Epsilon Aurigae is in many respects the history of astrophysics.”

And now that history is about to get a new chapter. Modern telescope technology should provide the sharpest and most detailed views of the Epsilon Aurigae system ever. Combined with measurements of the star’s dimming cycle collected by dedicated amateur observers, the latest eclipse may finally unmask the secrets of one of astronomy’s longest-running mystery shows.

The early observers

Epsilon Aurigae is a white supergiant star visible to the naked eye under moderately dark sky conditions. In 1821, German pastor and amateur astronomer Johann Fritsch noticed that Epsilon Aurigae had dimmed by a magnitude for about a year and then returned to its previous brightness. When the star dimmed again in 1848, and then again in 1876, astronomers realized they had something unusual on their hands.

The dimmings had occurred about 27 years apart. To modern astronomers, this would suggest a type of star system called an eclipsing binary, in which one of a pair of mutually orbiting suns periodically passes in front of the other, cutting off some or all of its light. To a distant observer, an eclipsing binary looks like a single star that shines steadily between periodic dips in brightness.

Nineteenth-century astronomers, however, labeled Epsilon Aurigae an “irregular variable” — a lone star changing in brightness. “Binary star theory just wasn’t that developed in the 19th century,” explains Robert Stencel of the University of Denver. Astronomers then were more familiar with variable stars and chose that phenomenon as their preferred explanation.

In 1903, the star dimmed a fourth time, precisely on schedule. Astronomers observed this one closely and found the star going through a slow 6-month decline in brightness, followed by a year-long flat minimum, and ending with a slow 6-month return to full brightness. It was then that astronomers finally realized for certain that this was not an irregular variable, but a strange and inexplicable eclipsing binary.

The length of the 1903 eclipse — the longest measured to date — left astronomers puzzled. Based on Epsilon Aurigae’s estimated distance of about 2,000 light-years, only a truly immense object would be able to produce a 2-year-long eclipse. Scientists estimated its radius as 3,000 times that of the Sun, making it by far the largest stellar-type object they knew of at the time.

The 1903 eclipse was also the first observed with the aid of spectroscopy. Splitting Epsilon Aurigae’s light into its component wavelengths (colors) revealed new mysteries about the unseen eclipsing body. During the entire eclipse, Epsilon Aurigae’s spectrum of colors did not change. All the invisible companion did was dim the light of the visible star equally across all wavelengths, much as a pair of sunglasses diminishes the brightness of a sunny day.

In addition, the eclipsing companion produced no visible spectral signature of its own, so it was completely dark. The 1928–1930 and 1955–1957 eclipses only confirmed the observations and deepened the enigma.