Uncovering the Secrets of the Great Supernova

Kamuela, HI – A once-in-a-lifetime nearby stellar explosion now unfolding in a neighboring galaxy has astronomers at the W. M. Keck Observatory scrambling to ask questions that can’t be answered at any other ground-based telescope in the world. The first big question: What causes this pivotally important type of stellar cataclysm?

Observing this spectacular supernova, dubbed PTF11kly, began on August 24, with the detection of the explosion in the nearby Pinwheel Galaxy, a.k.a. M101, by the automated Palomar Transient Factory (PTF) survey. That survey is designed to detect short-lived astronomical events as they happen.

Next, the brightening point of light was observed by the Grand Canary Telescope in the Atlantic and the star’s light was split into the first information-rich spectrum. Then, as the Earth turned and presented different telescopes to that part of the sky, the Lick Telescope in California got another spectrum of the exploding star, followed soon by a very high quality spectrum from the HIRES instrument on the Keck I telescope in Hawai’i.

Both Lick and Keck astronomers confirmed that the explosion is a Type Ia supernova – the kind that pop off occasionally in very distant galaxies. There has not been another Type Ia supernova this close to Earth in decades, and none have ever before been caught so early in the process of this type of stellar death.

“Nearby Type Ia’s are very rare,” said postdoctoral astronomy researcher Brad Cenko of the University of California at Berkeley.

Astronomers have long adored Type Ia supernovae because they seem to behave in a very predictable manner: brightening and reaching the same luminosity, before fading away. As a result when they happen in very distant galaxies, they are recognizable and can be used as “standard candles” to measure cosmic distances.

Such measurements led to one of the biggest cosmological discoveries of recent years: galaxies are moving further apart and the universe is expanding at an accelerating rate. That discovery, in turn, pointed to the existence of a sort of weird anti-gravity force astronomers call “dark energy.” 

“Given the importance of this supernova for both the Type Ia supernova distance scale and for constraining the progenitors of Type Ia’s, Keck Observatory responded rapidly and deployed assets to acquire both spectroscopy and adaptive optics imaging,” said Keck Observatory Director Taft Armandroff. Keck adaptive optics cancel out Earth’s atmospheric distortions to starlight.

Astronomers do not really know what causes Type Ia supernova, despite their importance. And that is why having one occur in a nearby galaxy and be studied so soon after its explosion began is so exciting for astronomers.

“Type Ia supernovae underlie one of the most important astronomical discoveries in the last few decades,” Cenko said. “But we still don’t know what their progenitor systems are.”

It’s generally believed that there are at least two stars involved in creating a Type Ia supernova. One star is most likely a white dwarf – a kind of dead star. What the other one is, no one is exactly sure. It could be another white dwarf, main sequence star (like our sun) or a red giant star.

One way to find out is look at high-resolution images of the Pinwheel Galaxy taken by the Hubble Space Telescope before August 25 and see if there was a star in the same location.

“Keck and Hubble are pretty well matched in terms of spatial resolution,” said Keck support astronomer Jim Lyke. “So we can do a direct comparison.” 

“We need very accurate images at very high resolution to match Hubble images,” agreed Lawrence Berkeley Laboratory astronomer Peter Nugent, who is the lead of the PTF Type Ia supernova program and chiefly responsible for the discovery of PTF11kly. The only way to do that is with Keck adaptive optics.

So in addition to gathering spectra of the supernova, astronomers started on the night of August 25 to take pictures of the supernova with the Keck II telescope adaptive optics system. Their hope is to get a very precise location of the star in the Pinwheel Galaxy so that they can look at those Hubble images and see if there was anything – like a red giant – there beforehand.

“If it was two white dwarfs it would be too faint to see,” said Cenko. Even if nothing is found in Hubble images, it will still be useful, he said, because it will put some limits on how large the stars could be to create a Type Ia supernova, and bolster the theory that two white dwarfs are the cause.

Another source of clues to the cause of the supernova are changes in the spectra as the explosion continues, Cenko explained. If, for instance, the companion star to the white dwarf was large, it would have likely shed lots of material in its final death throes. Then, when the explosion followed, that same material would be hit by the explosion itself. The shock waves of those collisions would create telltale signals in the spectra of the star as its explosion continues to expand into space. 

“Because we don’t know the progenitor system (of Type Ia supernovae) we don’t have a good grasp on how diverse the Type Ia class might be,” said Joshua Bloom, another UC Berkeley astronomer who is leading the Keck research team. “It is a bit troubling that we really don’t know.”

But with the advent of the Pinwheel supernova, hopes are high that a lot more will be learned about these cosmic yardsticks.

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The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Big Island of Hawaii. The twin telescopes feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system which cancels out much of the interference caused by Earth’s turbulent atmosphere. The Observatory is a private 501(c) 3 organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.