Mysterious Explosion Caused Massive Star to Brighten

September 10, 2008

Stars have onion-like layers that blow off in fiery explosions before a final killing blow—a supernova—turns them into black holes, according to a new theory of star death.

These repetitive blasts are too powerful to be caused by stellar winds, as previously believed—so they must come from a new type of explosion originating in the star's interior, astronomers say.

The theory was advanced this week in a study led by Nathan Smith, an astronomer at the University of California, Berkeley.

Using ground-based telescopes, Smith took a close look at the Homunculus Nebula, the cast-off material from the 1843 explosion of the nearby star Eta Carinae, the most luminous star in the Milky Way (see photo.)

That explosion suddenly and mysteriously brightened the star.

In the nebula, Smith discovered new, faster moving wisps of gas—too fast to be accounted for by stellar winds.

"We still don't know what the mechanism is that would initiate the explosion in the first place," Smith said, "but at least now we know that there was an explosion that we need to explain."

(Related: "'Brightest Supernova' Reveals New Kind of Star Death" [May 8, 2007].)

The new paper appears tomorrow in the journal Nature.

Double the Power

Researchers had already observed non-fatal explosions on other late-stage stars. Sometimes called "supernova imposters," the blasts are even less well understood than supernovae.

Using two telescopes at Cerro Tololo Inter-American Observatory and the International Gemini Observatory in Chile, Smith took another look at both the Homunculus Nebula and another shell of cast-off material, estimated to be a thousand years old.

(See a photo of another star that suddenly brightened.)

Most of the material from both blasts is crawling along at about 1.5 million miles per hour (650 kilometers per second). But Smith's newfound gas filaments are moving much faster, catching up to the materials from the thousand-year-old blast.

The finding potentially doubles estimates for the power of Eta Carinae's 1843 eruption, he said.

Not New?

Douglas Currie, an astrophysicist at University of Maryland at College Park, reported some fast-moving gas jets in Homunculus as far back as 2002, but he says the speeds reported in the new paper are significantly faster—up to double—than what has been seen before.

He doesn't agree that Smith's theory is necessarily new, since other researchers have previously suspected causes of the fast-moving gas other than stellar wind.

"I would call this a further and strong piece of evidence showing that it is a different kind of explosion," he said.

But study leader Smith said that the old speeds were on the edge of what could be produced by stellar winds—and the new speeds, even if only slightly faster—beget new questions.

He said the results "may change our interpretation of what happened in the 1843 event, and what that means for our understanding of the most massive stars."

"It means, essentially, that we still don't fully understand what is going on in the deep interiors of massive stars shortly before they die."




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