Star "Fireworks" Expected From Strange Eclipsing System

Ker Than
for National Geographic News
June 5, 2008
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.

(See a related photo of a March 29, 2006, solar eclipse as seen by the crew of the International Space Station.)

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.

Shrinking Orbit

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.

"Why it should be speeding up is not obvious," Stencel said.

"It's a simple extrapolation to say that if over the course of one decade we see this much change, then within a few decades some kind of punctuation may be coming along."

Observations during the last eclipse also suggest Epsilon Aurigae is shrinking by about half a percent a year.

Stencel thinks that a third object in the system—possibly a planet embryo, or planetesimal—is spiraling toward the supergiant or the stars at the center of the eclipsing object.

As its orbit shrinks, the planetesimal passes in front of Epsilon Aurigae more frequently, which could explain the shortening of the bright-dim cycle.

Eventually the planet embryo would crash into the star or its partner.

This would create bright flares like those seen in some types of interacting stellar binaries or recurring novas, where one star draws gas from its companion until it passes a critical mass limit and explodes.

Such flares in the Epsilon Aurigae system would cause a brightening visible to observers on Earth with the naked eye.

"That could pop it up to become one of the brightest objects in the sky," said Stencel, who presented his theory this week at a meeting of the American Astronomical Society in St. Louis, Missouri.

Public Invitation

Edward Guinan is an astrophysicist at Villanova University in Pennsylvania who was not involved in the study. He called Stencel's idea "plausible but unusual."

If the dark companion is receiving material from an external source like a planetesimal, it could reach a mass limit where it becomes unstable, Guinan said.

"The inner part of the disk could explode onto whatever is at its center," Guinan told National Geographic News.

New instruments that were unavailable during the system's 1982-1984 eclipse could allow astronomers to get their first glimpse of the dark object this time around.

Stencel is drafting proposals to gain observing time with NASA's Hubble Space Telescope and other instruments to focus on the system.

And he and his colleagues are urging public participation in the upcoming eclipse.

"We're trying to promote the idea that here's this singular event kind of on the scale of a Halley's comet," Stencel said.

"We invite people to observe it and report to their local science centers and help us collect some data."

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