National Geographic News
Images show a burst of light in the Carina Nebula.

The star system Eta Carinae (left) erupted in the 1800s, sending out light that has now been seen reflecting off dust clouds (right panels).

Images courtesy A. Rest, STSI/NASA/NOAO

Dave Mosher

for National Geographic News

Published February 16, 2012

For the first time, astronomers have glimpsed reflections of light from a "supernova impostor"—the so-called Great Eruption of the star system Eta Carinae, which was seen from Earth more than 150 years ago.

The new data should help scientists figure out what caused the stars to shine unusually brightly in the 19th century, as well as predict when the system's huge "ticking time bomb" star might actually die in a violent explosion.

Eta Carinae is a tumultuous system roughly 7,500 light-years from Earth.

At first astronomers though the object was a single massive star about 120 times heftier than the sun But in 2005 scientists discovered that Eta Carinae is actually two stars—a massive blue star and a smaller companion.

Before 1838 the blue star was even more massive. Then Eta Carinae mysteriously threw off unusually bright light for the next two decades in an event that became known as the Great Eruption. (See "Mysterious Explosion Caused Massive Star to Brighten.")

Eta Carinae's tantrum was dimmer than a supernova, and the star system survived intact. However, the large blue star shed material equal to more than ten times the mass of the sun, creating a thick nebula and two fiery lobes of illuminated gas and dust that now envelop the stars.

Using the National Optical Astronomy Observatory's Victor M. Blanco telescope in Chile, a team of astronomers recently captured "light echoes" from the Great Eruption bouncing off dust about a hundred light-years away from the outburst.

Eta Carinae "is one of the best studied objects in the Milky Way galaxy, but we've never had the chance to directly sample the light emitted from the eruption itself. Now we do," said study co-author Armin Rest, of the Space Telescope Science Institute in Baltimore, Maryland.

"These observations could help us understand what triggered the Great Eruption."

Eta Carinae a Strange Star System

The light echoes that Rest and his team sampled have added an unexpected twist to the tale: "The Great Eruption was significantly cooler than allowed by simple stellar-wind models used to explain supernova impostors," he said.

Stellar wind is the steady stream of charged gas that flows in all directions from stars. Models suggest that a supernova impostor expels such massive amounts of gas that the size of the star actually expands, causing the object to appear brighter and cooler.

(Related: "Star Caught Eating Another Star, X-Ray Flare Shows.")

But the light echoes show that the Great Eruption was too cool to fit stellar-wind models. What the discrepancy means for Eta Carinae is uncertain. Still, the team does have an idea of what might have caused the eruption in this particular system.

Eta Carinae's smaller companion star has a highly elliptical orbit around its larger star, Rest said. So at its nearest approach, the small star might have swung by close enough to help trigger the Great Eruption.

"Both stars have stellar winds that collide. When the small star gets close, it really disturbs this system," Rest said.

In fact, historic observations show that Eta Carinae ramps up in brightness about every five years. Such records line up with the small star's close passes near the larger star.

Such periodic eruptions are "like the last ... burp before [a massive star] kicks the bucket," Rest wrote in an email to National Geographic News. Eventually, Eta Carinae will certainly end up as a real supernova.

"These last stages, in particular events like the Great Eruption, are important to understand the path a star takes to go supernova."

(Also see "Supernova's Beginning Blast Shown in 3-D—A First.")

Taking More Snapshots of a Supernova

Rest and his team next plan to search for more light echoes in different directions from the original blast, to fully reconstruct what the Great Eruption looked like.

"Right now we just have one snapshot in time from one direction," he said.

Dissections of light, called spectral analyses or spectra, have already helped the team understand the temperature of the eruption and the composition of the star system at that time.

"In the future we hope to get spectra from different times and directions. We might be able to create an image of the Great Eruption as it evolved over time."

The study of Eta Carinae's light echoes was published online February 15 by the journal Nature.



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