Mystery of supernova birth points to white dwarfs

Astronomers have found the first direct evidence that some star explosions are triggered by compact stars called white dwarfs.

Scientists studying the youngest type of Ia supernova ever found worked backward to pinpoint its explosion time with unparalleled accuracy. In doing so, they confirmed that a white dwarf was the source of the blast, and gleaned insights into the nature of the dwarf's companion star.

The discovery occurred in August, when astronomer Peter Nugent spotted a surprising object while poring over data from the Palomar Transit Survey's robotic telescope at Palomar Observatory in Southern California. The object was quickly confirmed to be a type Ia supernova. High-resolution follow-up observations were made within hours by the Keck telescope in Mauna Kea, Hawaii, identifying the elements that burst from the blast.

The speedy response allowed Nugent and his team to follow the evolution of the supernova, called SN 2011fe. [Amazing Photos of Supernova Explosions]

Tracing backward

As the light of the explosion reached the brightness of 2.5 billion suns, then slowly faded, the team worked backward to determine exactly when the supernova occurred. Located only 21 million light-years from Earth in the Pinwheel Galaxy, the supernova is the closest one to our planet in 25 years. (A light-year is the distance light travels in a year, about 6 trillion miles, or 10 trillion kilometers.)

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

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.

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