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The Brightest Supernova Ever Observed Tests the Limits of Physics

This explosion blazes about 570 billion times brighter than our Sun, according to researchers

Colored images show the bright supernova as seen by two different telescopes (the Dark Energy Camera on the left and the Las Cumbres Observatory Global Telescope Network 1-meter telescope on the right). (The Dark Energy Survey, B. Shappee and the ASAS-SN team)
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Last summer, astronomers operating two telescopes in Chile spotted an exploding star—a supernova—that is so luminous and so powerful that it  "approaches the limits of what theorists believe is possible for these mighty cosmic outbursts," reports Lee Billings for Scientific American. The supernova is the brightest humans have ever known.

Astronomer Subo Dong, of the Kavli Institute for Astronomy and Astrophysics, and his colleagues first noticed the exploding star on June 14, reports Kate Becker for Nature, about nine days after the supernova's peak. The two telescopes they use are operated by the All Sky Automated Survey for SuperNovae (ASASSN) at the Cerro Tololo Inter-American Observatory in Chili. But immediately, the supernova started to seem odd.

At first, the team thought that the supernova's spectrum looked unlike any other that they had seen. Then, they realized that it could match another supernova observed in 2010 if the spectrum had red-shifted. This happens because it takes billions of years for light to travel from these distant explosions to Earth. In that time, the expansion of the universe itself stretches the wavelengths out, making them appear redder than they started.

But in order for the light this far away to ever reach our lenses, the newly observed supernova had to be incredibly bright. Even so, extreme supernovas aren't unknown. About a dozen superluminous supernova—each roughly 100 times brighter than typical supernova—have been observed in the past decade, Becker writes. This new one, dubbed ASASSN-15lh, was at least twice as the brightest previously observed, the researchers reported at the time of its discovery in arXIv.org.

Now the team gives more details about the supernova, in a paper recently published in the journal Science. ASASSN-15lh flared into magnificence in a galaxy about 3.8 billion light-years away and blazes about 570 billion times brighter than our Sun. It is also hotter than any other supernova observed. 

"If it were only as far away as Siruis, which at a distance of 8.6 light-years is the brightest star in the nighttime sky, it would blaze overhead almost as powerfully as the Sun," Billings writes for Scientific American. "If it were as close as Pluto, it would vaporize the Earth and all the other worlds in our solar system."

The source of this explosion is only about 10-miles wide, reports Kat Long for The Wall Street Journal. But exactly what lies at its center isn't yet clear. Dong suspects that it could be an enormous star, the kind hundreds of times as massive as our Sun. Those stars are very rare and poorly understood. However, if the supernova did come from this kind of star, the team should be able to see in the supernova's spectrum the signature of decaying nickel that was once forged in the heart of that giant.

Another possibility is that the explosion could have come from a magnetar—a rapidly spinning star with strong magnetic fields wrapped around it. For this explanation to be true, the magnetar would have to be spinning so fast that it completed a revolution every millisecond, a feat "most theorists believe is just barely possible," Billings writes for Scientific American

Still more observations are needed to really get to the heart of this luminous explosion. But the effort should be worth it. "Discoveries like this are the reason I am an astronomer," Benjamin Shappee of the Carnegie Institution for Science in California, tells the Associated Press. "Nature is extremely clever and it is often more imaginative than we can be."

Read more about the superluminous supernova in our in-depth coverage on Smithsonian.com.

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