for National Geographic News
A celestial threesome seems poised to create a stellar light show that should be visible from Earth within a few decades, astronomers announced this week.
At the heart of the drama sits Epsilon Aurigae, a supergiant F-type star that is considered one of the most unusual objects in the sky.
Even though it is about 242 million miles (389 million kilometers) wide, Epsilon Aurigae is totally eclipsed every 27 years by an even bigger disk-shaped neighbor.
Astronomers don't know what the star's colossal partner is, but one leading theory suggests it's a gas cloud more than a thousand times as wide as the sun that harbors two small stars at its core.
Alternatively the object at the center of the cloud may be a black hole or a single larger star that has somehow siphoned gas from Epsilon Aurigae to create an enveloping cloak of darkness.
Each eclipse lasts nearly two years, by far the longest in any known binary star system. The next eclipse starts in August 2009 and should run through May 2011.
But marked changes in the behavior of the massive star and its enigmatic companion suggest that a third object is about to crash the affair.
"It has prospects to produce a bit of fireworks by mid-century," said Robert Stencel, an astronomy professor at the University of Denver.
Epsilon Aurigae is a variable star, which means it has a cycle of brightening and dimming.
But this cycle has sped up in recent years, from 96 days a decade ago to about 67 days now.
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