Visible echoes reprise 19th century spectacle

Light from long-ago stellar eruption holds clues to its cause

Scientists are closer to understanding an enormous two-decade-long eruption that transformed one of the galaxy’s most massive stars into a fireball millions of times brighter than the sun.

LEFTOVER LIGHT After analyzing light echoes left over from an eruption of the binary star Eta Carinae more than 150 years ago, scientists suggest the spectacle was created by an explosion rather than an energetic stellar wind. NASA, ESA, STScI

From 1838 to 1858, astronomers watched the binary giant star Eta Carinae erupt, shedding more than 10 solar masses of material and producing an oddly shaped, double-lobed cloud 7,500 light years from Earth. Scientists have thought a dense stellar wind fueled Eta Carinae’s outburst, and considered it the prototype for “supernova impostors,” or shorter-lived eruptions that don’t quite destroy a star.

But new observations of Eta Carinae’s Great Eruption suggest an explosion may have been its cause, says study coauthor Armin Rest, an astrophysicist at the Space Telescope Science Institute in Baltimore, Md. While scientists already knew much about the star and its current state, Rest says they were missing information about the outburst itself, originally observed without the aid of modern technology.

So Rest’s team sifted through dust clouds near Eta Carinae, clouds that form a sort of net that captures and bounces the eruption’s original light back to Earth in what’s known as light echoes.

These echoes act like time capsules that preserve the energy spectrum of the light as it would have appeared in the mid-1800s. “It is like a science fiction dream to be able to travel in time to the past and review and analyze the Big Eruption with 21st century instrumentation,” says Augusto Damineli, an astrophysicist at the University of Sao Paolo. 

Rest says he didn’t expect to find light echoes from Eta Carinae, but he and his colleagues were eventually able to capture the delayed reflections over eight years. 

The spectrum bouncing off the dust clouds didn’t match predictions drawn from accepted stellar wind theory, Rest and his colleagues report in the Feb. 16 Nature.

Most surprisingly, the temperature indicated by the light echoes was about 2,000 degrees Celsius cooler than anticipated, meaning the stellar zephyrs corresponded to a balmy 4,700 degrees. The light echoes suggested that something other than windblown starstuff, such as a mini-explosion or an interaction between Eta Carinae and its companion played a role in the eruption.

“The result is really a surprise, in the sense that the Big Eruption was thought to be a kind of very dense wind ejection, not an explosion,” Damineli says.

But some scientists are skeptical of the explosion interpretation, suggesting the discrepancy in temperature alone isn’t enough to overthrow the classical stellar wind theory. “The spectrum that they describe is, in fact, reasonably consistent with what we expected based on theory,” says Kris Davidson, an astrophysicist at the University of Minnesota in Minneapolis.

Others accept that something odd must be going on, and say the eruption reveals the many possibilities at work when a star nearly dies. “I don’t think this paper establishes that it’s some kind of freak event,” says Stan Owocki, an astrophysicist at the University of Delaware in Newark. “I think it could be an extreme version of these phenomena.”

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