ST. LOUIS — Already bright enough to be seen with the naked eye, the star epsilon Aurigae may be trembling at the brink of a powerful outburst. The fireworks, which would be easily visible in the Northern Hemisphere, could take place by midcentury, reported Robert Stencel of the University of Denver on June 3 at a meeting of the American Astronomical Society.

The visible star is part of a binary eclipsing system that lies about 2,000 light-years from Earth. Every 27 years the body is hidden by a partner that passes in front of it. As the two bodies orbit each other, it is relatively simple to measure how the naked-eye star’s size, temperature and brightness changes. Estimates for the mass of the star range from 12 to 15 times the mass of the sun.

But the epsilon Aurigae system is no garden-variety binary. Its bizarre properties have puzzled astronomers for nearly two centuries.

For starters, its partner has never been seen. And because the eclipse lasts for nearly two years, the partner must be about as gigantic as the visible object in both size and mass — the equivalent of 12 to 14 suns. Yet it has never been directly imaged. Models suggest the unseen body is a huge dark disk of gas and dust, requiring a massive central object within the disk to stabilize it.

Observations indicate that when epsilon Aurigae is not in eclipse, it undergoes quasiperiodic variations in brightness that have sped up over the past two decades, from a period of 95 days in the 1980s to 67 days now. In addition, studies during the last eclipse, from 1982 to 1984, suggest that the visible star is shrinking its radius at the rate of half a percent a year.

Should these changes continue to accelerate, Stencel proposes, the star will be ripe for a cataclysmic explosion some time in the next few decades. It’s unclear, he notes, if the star would blow to smithereens or if it would simply exhibit a major flareup.

Both professional and amateur astronomers are now gearing up for 2009, when the star once again undergoes a long-term eclipse by its unseen partner. A planned worldwide campaign could shed new light on the system and its fate. Imaging interferometers such as CHARA (Center for High Angular Resolution Astronomy), a six-telescope array on Mount Wilson in California that can measure minuscule changes in the star’s shape, could be particularly useful, Stencil says.

“There is no question that some relatively rapid changes appear to occur in this system,” says theorist Mario Livio of the Space Science Telescope Institute in Baltimore. But “the system is not sufficiently well understood to be able to make reliable predictions. At the same time, there is no question that the system should continue to be monitored in the coming years, in the hope that the refined observations will allow for a better understanding of the physical processes that are taking place.”

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