The constellation is easy to find. Just after dark, look high in the west for the yellowish star Capella, its brightest star. The star, the sixth brightest in the sky, anchors a rough pentagon of stars, which is how Auriga is traditionally depicted. Capella's name comes from a Roman word that means "she-goat.'' Yes, the proud Charioteer has a goat perched on his shoulder. To make matters even stranger, just to the west of the pentagon (below Capella) is a small triangle of stars called "the Kids.'' Auriga is apparently burdened with the she-goat's three children, as well.

The star called Epsilon, farthest to the right in the triangle, is the strangest kid of all. Epsilon is a variable star, which means that its brightness varies according to a predictable pattern. Variable stars are not that uncommon in the Milky Way galaxy. What makes Epsilon the strangest star in the sky is the period of its variability. Every 27 years, it loses brightness for two years, dimming to about half its normal brightness and then slowly brightening to full luminosity again.

In part, the explanation is simple. Epsilon must have a mysterious, unseen companion, a star that orbits the main star and every 27 years passes in front of it along our line of sight. As a result, Epsilon dims considerably.

Such "eclipsing binary stars'' are not that unusual, but a two-year dropoff is a tad hefty. What gives with Epsilon?

To start with, the visible star in the Epsilon system is enormous. Astronomers call such stars supergiants, and Epsilon's might be 220 million miles wide. That's about the size of the orbit of Mars around our sun.

The star that orbits Epsilon must be considerably larger to dim the main star for two whopping years -- about 2 billion miles wide. Let's face it: A star the size of the orbit of Saturn around the sun strains credulity.

One clue to the mystery is that Epsilon doesn't dim all that much. That suggests that the companion isn't a star at all but a disk-shaped collection of dust and gas, which orbits the main star.

Sadly, the disk hypothesis doesn't explain everything. Such a disk usually has a star at its center. As that star blocks Epsilon during the middle of the eclipse, Epsilon briefly should get very dim.

That's not what happens. Instead, during the middle of the eclipse, Epsilon suddenly brightens. The disk orbiting Epsilon appears to have a hole at its center.

Some astronomers believe that the disk has at its center not one star but two that orbit each other, like thumbs twiddling. The action of the two stars clears out a hole in the center of the disk and allows a bit more of Epsilon's light to shine through.

Even this three-star system has its problems. Because of the behavior of Epsilon, astronomers assume the disk is there, but no one has yet observed it directly.

Eventually, astronomers will figure out what is happening to Epsilon Aurigae, but the very nature of the star's variability works against them. The last time the star dimmed was between 1982-1984. They will have to wait until the star dims again beginning in 2009 to gather more data about this mysterious stellar system.

Tom Burns directs Ohio Wesleyan University's Perkins Observatory in Delaware.