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supernova

Shockwaves amplify super-bright star explosions

Astronomers now have proof for a theory that explains why super-luminous novae and some other astronomical explosions are brighter and more powerful than science can explain: powerful shockwaves amplify the explosions beyond any traditional scale for nuclear explosions.

In a typical year, there are around 50 novae, nuclear explosions on the surface of white dwarf stars, in our galaxy.

“Astronomers have long thought the energy from novae was dominated by the white dwarf, controlling how much light and energy are emitted,” says Laura Chomiuk, an astronomer at Michigan State University and study coauthor. “What we discovered, however, was a completely different source of energy—shockwaves that can dominate the entire explosion.”

novae shockwaves
Shockwaves are the secret to novae’s intensity, new research suggests. (Credit: Michigan State)

As the explosion begins, it ejects a cooler, slower wave of gaseous material, relatively speaking. Behind it, though, is a hot, fast wave speeding right behind it. The collision of the two ejections produces a shockwave, which results in a spectacular explosion of heat and light.

“The bigger the shock, the brighter the nova,” Chomiuk says. “We believe it’s the speed of the second wave that influences the explosion.”

“Novae are little laboratories in our galactic backyard that we can use to study some of the most luminous explosions in the universe…”

Now that the theory has been proven, astronomers use novae to better understand other super-charged explosions, like those that mark the death of massive stars in galaxies far away.

“Novae are little laboratories in our galactic backyard that we can use to study some of the most luminous explosions in the universe,” Chomiuk says. “As future novae happen, we’ll be able to observe them to better understand how shocks light up and fuel explosions. We really want to find out how common and energetic shocks are.”

gamma ray nova
A team of scientists has found that gamma rays are emitted from a stellar explosion known as a nova. Where and how these high-energy waves are produced has been a mystery among astronomers. In this artist’s rendering, the yellow depicts the material that is expelled just days after the stellar explosion. (Credit: Bill Saxton, NRAO/AUI/NSF via Michigan State)

When novae happen, there’s a good chance they’re being observed by Ohio State University’s All Sky Automated Survey for SuperNovae. ASAS-SN is a suite of robotic telescopes in the northern and southern hemispheres. In fact, ASAS-SN discovered this particular nova featured in this study—ASASSN-16ma.

When ASASSN-16ma was discovered in October 2016, Kwan-Lok Li, Michigan State astronomer and lead author, and Chomiuk, requested a second telescope—NASA’s Fermi Gamma-ray Space Telescope—also observe the explosion and measure the gamma rays the nova released. Gamma rays are a direct tracer of shockwaves.

Chomiuk also alerted the American Association of Variable Star Observers. This international band of professional and amateur astronomers joined in and provided much of the optical data that proved this discovery.

The observations collected from these sources provided data that was unparalleled. The unequivocal results, when expressed through a graph, display a pulsating line of energy—resembling a galactic heartbeat, of sorts—with the optical light and the gamma rays mirroring each other and proving the theory. The gamma rays and the optical light come from the same source—showing that shocks dominate the nova’s luminosity.

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“The nova’s brightness and how strong our data were really surprised me,” Li says. “Other novae may take days or weeks for us to collect sufficient data. This one, though, was visible after just one day, and we knew it was a good one.”

The study appears in the journal Nature Astronomy.

Additional astronomers contributing to this study are from Columbia University, Tartu Observatory (Estonia), University of Pittsburgh, Carnegie Observatories, Ohio State University, Universidad Diego Portales (Chile), Millennium Institute of Astrophysics (Chile), American Association of Variable Star Observers, University of Western Australia, and Variable Star Observer’s League in Japan.

Source: Michigan State University

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