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Exploding star yields its secrets


Radiation from an exploding star (depicted as squiggly white lines, detail at right) lit up a relatively dense shell of gas that had been shed by the star in its last days.

Adapted from Ofer Yaron

By Sid PerkinsFeb. 13, 2017 , 11:00 AM

Millions of years ago in a nearby galaxy, a giant star blew up. Just a few hours after the light from this supernova reached Earth, astronomers had trained a slew of telescopes at the blast, giving them unprecedented insight into the immediate aftermath of these cosmic explosions. The findings, published today, are providing astrophysicists with new information about how these events cast starstuff into the cosmos, elements that pepper subsequent generations of stars and are essential for the formation of planets—and any lifeforms that may live on them.

Astronomers at California’s Palomar Observatory first detected the supernova, dubbed SN 2013fs, in a galaxy about 160 million light-years from Earth on 6 October 2013. Less than 3 hours later, a team led by Ofer Yaron, an astrophysicist at the Weizmann Institute of Science in Rehovot, Israel, had gleaned follow-up observations in ultraviolet and X-ray wavelengths, among others. Analyses of those spectra suggested that the star had exploded no more than 6 hours earlier, making them the earliest such detailed observations of a supernova ever made, the researchers report online today in Nature Physics.

The star that produced SN 2013fs was a so-called red supergiant and was probably between 8 and 10 times the mass of our sun and no more than a few million years old before it exploded, Yaron says. That a star that size blew up in a supernova isn’t surprising; current astrophysical models suggest that all such stars do. But the team’s detailed observations did yield a big surprise—the star appeared to be surrounded by a relatively dense shell of gas shed by the star during its last days.

“The star had substantial mass loss in the last year of its life,” says Derek Fox, an astronomer at Pennsylvania State University in University Park who was not involved in the new study. “That’s new.”

As radiation spewed forth from the supernova, it lit up the gas surrounding the star and stripped electrons from atoms there. When those electrons recombined with other atoms, they gave off light at specific wavelengths that let the researchers identify the materials in the shell, including oxygen, helium, and nitrogen—atoms that had previously been forged by fusion reactions in the outer layers of the star. Emissions at those wavelengths faded about 20 hours after the explosion, Yaron says.

That time span gave the team an idea of the size of the shell: Its outer fringe was about 5 times the distance from the star as Neptune is from our sun. Presuming that the material was previously shed at a speed of about 100 kilometers per second, the findings suggest that the gaseous shroud of material had been emitted from the star during the previous 500 days. As shock waves from the explosion ripped through the shell of gas near the star, the material was heated to temperatures of up to 60,000°C, the team reports. Over the course of 5 days, that shell of material was completely swept away by the supernova’s explosion.

The researchers estimate that the shell of gas around the star held about one-thousandth the mass of our sun—which sounds like a small amount but is a little more than the mass of Jupiter and is much more than most scientists presume should have been present. “There’s a good bit of material where it shouldn’t be according to most models of stellar evolution,” says Adam Burrows, an astrophysicist at Princeton University.

These data will give astrophysicists new insights into a phase of stellar evolution that previously was murky. That’s because detailed observations of supernovae usually don’t occur before the exploding star destroys evidence of its nearby environment, Yaron says.

As current and future surveys of the night sky pick up their observational pace, more and more supernovae will be caught early in the act of exploding, says Norbert Langer, a theoretical astrophysicist at the University of Bonn in Germany. That, in turn, will let scientists know whether supernova 2013sf was a statistical fluke or a run-of-the-mill exploding star. Then, astronomers might even have the data needed to spot a supernova before it blows its top.  

Source: Science Mag