Image: Star
NASA/ESA/A. Gal-Yam, Weizmann Institute of Science, Israel
These photographs from NASA's Hubble Space Telescope have been used to uncover the progenitor star to a supernova that exploded in 2005. To the surprise of astronomers, the progenitor is a rare class of ultra-bright star that, according to theory, shouldn't explode so early in its evolution.
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updated 3/23/2009 1:42:31 PM ET 2009-03-23T17:42:31

A neat piece of detective work to trace the roots of an exploded star has turned up a deeper mystery: why the victim, which turned out to be a beast of star more than 50 times bigger than our sun, didn't disappear into a black hole, as prevailing theories of stellar evolution predict, and why it exploded in the first place.

The story begins with a dead body — the remains of an exploded star spotted in the galaxy NGC 266 on Oct. 5, 2005. It was designated SN (for supernova) 2005gl.

Astronomers decided to comb through archived images taken by the Hubble Space Telescope looking for a "before" shot. They got lucky, pinpointing an extremely bright blue star that appeared perfectly positioned at the scene.

They then used Hubble to take an "after" shot in 2007, once the supernova had faded from view. The star was gone, providing proof that SN2005gl was its progeny.

The case was hardly closed, however.

"It's just surprising from conventional theory that such a star exploded," said astronomer Douglas Leonard, with San Diego State University in California.

Typically, stars that are about eight to 20 times more massive than the sun will end in a supernova explosion. Bigger stars, theory predicts, turn into black holes.

But something different happened to NGC266_LBV 1. It apparently exploded during or shortly after a violent eruption phase, with most of its hydrogen-rich outer shell still intact and without a massive iron core of nuclear fusion ash that is presumably a prerequisite for a supernova blast, report Leonard and Avishay Gal-Yam, with Israel's Weizmann Institute of Science, in this week's Nature.

"Maybe the star was part of a binary system that ingested its companion and that stirred things up in the core and made the star explode," said Leonard. "That's just sort of a wild idea. Or maybe the stellar evolution theory is not correct and maybe stars can explode at different points in their evolution."

The star in question was known as a luminous blue variable, similar to Eta Carinae, a massive and highly unstable star in our own Milky Way galaxy.

"LBVs are a bit like volcanoes in the sense that these eruptions can occur over and over again, and the star can sometimes be dormant for many years," said University of California at Berkeley astronomer Nathan Smith, who previously proposed that the well-known supernova 1987A was caused by a luminous blue variable star.

"By actually detecting the progenitor star as an object that appears to have been an LBV before it exploded, (Gal-Yam and collaborators) provide direct evidence that LBVs can in fact explode as supernovae," Smith wrote in an e-mail to Discovery News. "This...firms up the notion that in some cases LBVs, or stars masquerading as them, do indeed explode as some of the supernovae we see."

© 2012 Discovery Channel

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