Brightest supernova ever seen discovered, scientists say

Scientists say they have discovered the brightest, most energetic, and likely most massive supernova -- a powerful and luminous stellar explosion ever identified. SN2016aps may have formed as the result of the merging of two massive stars prior to the explosion, according to researchers, including those from Harvard University in the US.

The explosion energy of SN2016aps was ten times that of a normal-sized supernova, they said. "SN2016aps is spectacular in several ways," said Edo Berger, Harvard University professor and co-author on the research paper published in the journal Nature Astronomy.

"Not only is it brighter than any other supernova we have ever seen, but it has several properties and features that make it rare in comparison to other explosions of stars in the universe," Berger said. The team first identified the supernova in 2016 using data from the Panoramic Survey Telescopes and Rapid Response System (Pan-STARRS).

This transient astronomical event occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. A four-year follow-up study tracked its slow evolution and significant energy release.

Archival images retrieved during the study revealed a rising light curve dating back to December 2015, allowing the team to better understand the nature and explosion of the supernova. In a typical supernova, radiation in visible light accounts for just one percent of the total explosion energy.

In SN2016aps, the explosion energy is unprecedented, and the supernova radiated energy that makes it outshine normal supernova explosions by 500 times, the researchers said. "The intense energy output of this supernova pointed to an incredibly massive star progenitor. At birth, this star was at least 100 times the mass of our Sun," said Berger.

Scientists don't believe the explosion got that big on its own. "Spectroscopic observations during the followup study revealed a restless history for the progenitor star," said Matt Nicholl, of the University of Birmingham in the UK and lead author of the study.

"We determined that in the final years before it exploded, the star shed a massive shell of gas as it violently pulsated. "The collision of the explosion debris with this massive shell led to the incredible brightness of the supernova. It essentially added fuel to the fire," Nicholl said.

SN2016aps also held another surprise for scientists: high levels of hydrogen gas. Massive stars typically lose the majority of their hydrogen to stellar winds long before they begin pulsating.

"That SN2016aps held onto its hydrogen prompted us to theorize that two less massive stars had merged together since lower mass stars hold onto their hydrogen for longer," said Berger. "The new star, borne of the merger, was heavy with hydrogen and also high enough in mass to trigger pair instability," he said.

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