Theory Links Supernovae To Smaller Set of Outbursts

Photo: Eta Carinae, shown surrounded by its expanding nebula of gas and dust, underwent and survived an explosion in 1843 that Smith believes is an indicator of the star's impending death.
Nathan Smith, NASA/Courtesy
Eta Carinae, shown surrounded by its expanding nebula of gas and dust, underwent and survived an explosion in 1843 that Smith believes is an indicator of the star's impending death.

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A new discovery about the biggest and brightest star in the galaxy may change scientists' understanding of how massive stars reach the ends of their lives.

Scientists have been stumped by Eta Carinae ever since its outburst in 1843, when it lost 10 times the mass of the sun but survived the explosion.

Now this event can be explained as an example of a smaller, fainter type of explosion that precedes the final supernova death, said Nathan Smith, a UC Berkeley postdoctoral student who published his theory in the latest issue of Nature magazine.

"We have to start thinking differently than we did before," Smith said. "This is something that may be happening in massive stars shortly before they die in a real supernova, maybe a kind of precursor event."

Smith was first alerted to this possibility when he was studying Eta Carinae in Chile and saw high-velocity material from the 1843 outburst traveling away from the star, he said.

Before that, it was thought that the outburst was a result of stellar winds pushing material off the star. But the material, which was traveling at speeds between 3,000 and 6,000 kilometers per second, was too fast to have come from wind on the star's surface, he said.

"That meant the cause was something deep within the star, rather than on the surface," Smith said.

In fact, the material was moving so quickly that it began colliding with material from an older eruption that occurred around a thousand years ago. This earlier eruption was probably in the same class of explosion, he said, which suggests that there may be a series of smaller outbursts before a star dies.

Similar explosions, typically 10 times as faint as an actual supernova, have been spotted in the Milky Way and nearby galaxies, Smith said. If his theory is true, these explosions may be a characteristic early warning sign that a massive star is close to dying.

Stan Owocki, a professor of astronomy at the University of Delaware who specializes in the mass loss of luminous stars, said Smith's work has the potential to be very important to the field of astronomy.

"The thing that's significant about this result is that it suggests that the eruption of Eta Carinae is more related, perhaps, to supernova explosion from the deep interior than it is to superwinds from the surface," Owocki said.

Owocki, who authored a paper in 2004 supporting the theory that Eta Carinae's outburst was caused by stellar winds, said he has no hard feelings about the new findings just because they contradicted his initial research. In 2006, Owocki and Smith published a joint paper on the importance of stellar eruptions in the lives of stars.

"Usually when you have an explosion it blows up the star, and that's the end of it. But in this case we seem to have pre-explosions that cause the star to shed a fraction of its mass, but not be totally destroyed," Owocki said.

Supernovae, as well as the kinds of explosions Smith is studying, only occur in stars that are at least 20 times as massive as our sun, Smith said.

He said the long-term goal of his research is to find out how much longer a star has to live when it experiences an explosion like that of Eta Carinae, meaning he will have to look farther than just nearby stars.

"This star, Eta Carinae, is the closest and best studied example we have, but it is just one of a diverse class of outbursts," he said. "In order to understand the broader significance of these explosions, we need to take a closer look at similar events in other galaxies."


Rachel Gross covers research and ideas. Contact her at [email protected]

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