The original star behind a relatively nearby supernova explosion has finally been identified as a white dwarf. The result is an important step in confirming theories about this type of stellar blast.

The supernova in M101. Credit: BJ Fulton/LCOGT

Supernova SN 2011fe went off on 24th August 2011 in the Pinwheel Galaxy, known as M101 to amateur astronomers. It lies 21 million light years away, and is the closest, and therefore the youngest, type Ia supernova ever discovered. Johua Bloom, Assistant Professor of Astronomy at University of California, Berkeley, called it “the supernova of a generation” at the time. When SN 2011fe reached its peak 20 days later its brightness was 2.5 billion times that of our own Sun.

“A fortuitous observation only four hours after we think the star exploded allowed us to put much more constraining limits on the size of the thing that blew up,” says Bloom. The measured density and size of the progenitor meant it couldn’t be anything else other than a white dwarf; a body the size of Earth but containing the mass of the Sun. This makes them extremely dense – a teaspoonful of white dwarf material would weigh a tonne.

Naked white dwarfs are the leftover cores of dead stars. The consensus model is that in a type Ia supernova, the white dwarf collects material from a stellar companion with its high gravitational field. The material accumulated briefly turns the white dwarf into a small star again as it heats up and undergoes thermonuclear fusion. However, the white dwarf can’t regulate fusion in the way a normal star can and a runaway reaction ensues, tearing the white dwarf apart.

But evidence supporting this model has been sparse. Modern computational and search techniques ensured that full advantage was taken of this new opportunity. After submitting papers to the journal Nature in November 2011, astronomers from UC and Lawrence Berkeley National Laboratory (LBNL) got confirmation of their conclusion that SN 2011fe’s progenitor – its original star – was a white dwarf.

After the initial explosion was spotted by the Palomar Transient Factory survey, a deeper, wide-field image of the Pinwheel Galaxy was obtained by the Physics Innovations Robotic Telescope Explorer (PIRATE), a teaching telescope which is operated by the UK’s Open University. Neither of these instruments could see the progenitor, or any shock waves or resulting hot gas. And nor, as it happened, could the Hubble Space Telescope.

Daniel Kasen and Ken Shen, from UC and LBNL respectively, were able to use all of these results to do some detective work that ruled out large progenitors. The exploding star had to be a white dwarf. As Kasen says, “The earlier you get on it (the supernova), the better your chance of actually seeing the glowing outflow from the shock wave.” No one has yet seen a shock wave glow from a type Ia explosion. “If it had been 10 instead of 20 million light-years from Earth, we might have seen something four hours after the explosion,” Kasen adds.

The results were presented by Bloom at the 219th American Astronomical Society meeting In Austin, Texas, and have been published in the 10th January issue of Astrophysical Journal Letters.

The institutions involved with this paper, Alongside UC and LBNL include: UC Santa Barbara, Arizona State University, the Open University, Max Planck Institute for Extraterrestrial Physics, Astronomical Observatory of Mallorca, and Oxford University.