What is a Champagne Supernova Astronomy?

PASADENA, Calif.- An international team of astronomers at the California Institute of Technology, University of Toronto, and Lawrence Berkeley National Laboratory have discovered a supernova more massive than previously believed possible. This has experts rethinking their basic understanding of how stars explode as supernovae, according to a paper to be published in Nature on September 21.

The lead author of the study, University of Toronto postdoctoral researcher Andy Howell, identified a Type Ia supernova, named SNLS-03D3bb, in a distant galaxy 4 billion light years away that originated from a dense evolved star, termed a "white dwarf," whose mass is far larger than any previous example. Type Ia supernovae are thermonuclear explosions that destroy white dwarfs when they accrete matter from a companion star.

The discovery was made possible through images taken as part of a long-term survey for distant supernovae with the Canada France Hawaii Telescope. Follow-up spectroscopy led by Richard Ellis, Steele Family Professor of Astronomy at Caltech, with the 10-meter Keck Telescope was key to determining the unusually high mass of the new event.

Researchers say the surprisingly high mass of SNLS-03D3bb has opened up a Pandora's box on the current understanding of Type Ia supernovae and, in particular, how well they might be used for future precision tests of the nature of the mysterious "dark energy" responsible for the acceleration of the cosmic expansion.

Current understanding is that Type Ia supernova explosions occur when the mass of a white dwarf approaches 1.4 solar masses, or the "Chandrasekhar limit." This important limit was calculated by Nobel laureate Subramanyan Chandrasekhar in 1930, and is founded on well-established physical laws. Decades of astrophysical research have been based upon the theory. Yet somehow the star that exploded as SNLS-03D3bb reached about two solar masses before exploding.

"It should not be possible to break this limit," says Howell, "but nature has found a way! So now we have to figure out how nature did it."

In a separate "News & Views" article on the research in the same issue of Nature, University of Oklahoma professor David Branch has dubbed this the "Champagne Supernova," since extreme explosions that offer new insight into the inner workings of supernovae are an obvious cause for celebration.

The team speculates that there are at least two possible explanations for how this white dwarf got so fat before it went supernova. One is that the original star was rotating so fast that centrifugal force kept gravity from crushing it at the usual limit. Another is that the blast was in fact the result of two white dwarfs merging, and that the body was only briefly more massive than the Chandrasekhar limit before exploding.

Since Type Ia supernovae usually have about the same brightness, they can be used to map distances in the universe. In 1998 they were used to make the surprising discovery that the expansion of the universe is accelerating. Although the authors are confident that the discovery of a supernova that doesn't follow the rules does not undermine this result, it will make them more cautious about using them to measure distance in the future.

Ellis summarizes: "This is a remarkable discovery that in no way detracts from the beautiful results obtained so far by many teams, which convincingly demonstrate the cosmic acceleration and hence the need for dark energy. However, what it does show is that we have much more to learn about supernovae if we want to use them with the necessary precision in the future. This study is an important step forward in this regard."

Peter Nugent, a staff scientist with the scientific computing group at Lawrence Berkeley National Laboratory, is a co-author of the Nature paper. ###

Contact: Andy Howell, department of astronomy and astrophysics, University of Toronto (416) 946-5432 [email protected]

Richard Ellis, division of physics, mathematics, and astronomy, California Institute of Technology (626) 676-5530 [email protected]

Jill Perry, Caltech Media Relations (626) 395-3226 [email protected]

Visit the Caltech Media Relations website at http://pr.caltech.edu/media.

The earliest moments of a supernova – the cataclysmic explosion of a massive star – have been observed in unprecedented detail, in a development researchers say could help us better understand what happens to stars when they die.

Using data collected from Nasa’s Kepler space telescope in 2017, astrophysicists recorded the initial light burst from a supernova as a shockwave blasted its way through a star.

In a study published in the Monthly Notices of the Royal Astronomical Society, scientists suggested the star that exploded was likely a yellow supergiant, which is more than 100 times bigger than our sun.

Patrick Armstrong, a PhD student at the Australian National University and the study’s first author, said the earliest phase of a supernova had not ever been fully observed before.

“In order to capture this, you have to be looking at the right part of the sky, at the right time, with the right amount of detail, to be able to see everything,” he said.

Armstrong said the supernova, called SN2017jgh, was more than one billion light years away from Earth. “The light we were seeing had actually left that star a billion years ago.”

On average, astronomers expect one star to explode per galaxy every 100 years. “There are millions of galaxies in the night sky, which means depending on how good your camera is, you might get about one supernova a week or up to one supernova a day if you’ve got a good camera like the Kepler space telescope,” Armstrong said.

A supernova explodes rapidly but it takes weeks or months to brighten and then eventually dim. The early phase of its explosion is observable for only a few days.

The scientists made the discovery based on a “shock cooling light curve”, which measured the change in the amount of light emitted by the supernova over time.

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“We see in the night sky this tiny point of light get brighter and brighter … as the supernova explodes, and [then] get dimmer,” Armstrong said. “This is the first time we’ve ever seen the shock cooling light curve in complete detail.”

The spectrum of light released by the supernova also gave clues as to its composition.

“We take the light from that supernova and we split it up into [a] rainbow, and depending on what colours we see – if there’s lots of red or green – that can give us information about what elements are in that supernova,” Armstrong said.

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Armstrong said the observation allowed scientists to better understand what stars explode into different supernovae. “Normally we can’t get much information about these stars because they have exploded and there’s not much left to look at.”

Unlike other telescopes which take observations once daily, Nasa’s Kepler telescope captured images once every half an hour, enabling the light curve to be comprehensively documented. Kepler’s mission officially ended in 2018 when it ran out of fuel.

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