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Mystery of supernova birth points to white dwarfs

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"We were able to pinpoint the explosion time very accurately, to an uncertainty of just twenty minutes," Nugent, of Lawrence Berkeley National Laboratory, told

Knowing how much energy the supernova put out allowed the researchers to, in effect, rewind the stellar explosion to see how it began. Measurements of elements such as nickel enabled them to put a lower limit on the size of the source.

The astronomers also found large clumps of fast-moving carbon and oxygen that disappeared within hours.

"The early observations helped us to constrain the explosion really accurately," Nugent said.

With their results, the team was able to conclude that the cause of the supernova was an extremely compact star known as a white dwarf.

White dwarfs are small, dense stars whose Earth-like radius encompasses a sun-like mass. The core of a white dwarf is too cool to undergo fusion, so its energy slowly dissipates into space.

Astronomers have long suspected that these remnants of dead stars were the source of type Ia supernovae, but SN 2011fe provides the first direct evidence.

Searching for the second

A firsthand examination of the light from the supernova also revealed information about the celestial body that once orbited the white dwarf.

In a type 1a supernova, material flowing from a second star onto the white dwarf overloads the compact relic and triggers the blast.  The companion could be anything from a large red giant star to another white dwarf.

When stars explode, a shockwave rushes outward. Collisions with material around it cause the region to flare brightly. By studying the light from SN 2011fe, Nugent's team was able to rule out specific types of stars as the companion.

The neighbor star could not have been a red giant, Nugent explained, because collisions between the debris and a large, massive star would have been very obvious. The resulting light would have been several magnitudes brighter than what was detected.

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