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Supernova in distant galaxy may be the brightest and most powerful ever seen

And no one is quite sure what's fueling it

Beijing Planetarium / Jin Ma

Astronomers have spotted what is perhaps the most powerful supernova ever discovered, in a distant galaxy billions of light-years away. Dubbed ASASSN-15lh, it's a cosmic explosion about 200 times more powerful than the average supernova. And it's incredibly luminous, shining 20 times brighter than all of the stars in the Milky Way Galaxy combined. The supernova is so unequaled in scope that it could completely change how scientists think about such phenomena in the future.

Because this stellar eruption is so bright and powerful, astronomers think a very rare type of star may be at the heart of it: a magnetar. It's a super dense, collapsed star — known as a neutron star — that spins unbelievably fast, producing an intense magnetic field. The magnetar's rapid spin may be providing the energy the supernova needs to produce such spectacular brightness for a sustained period of time. Think of a hand crank that powers a light bulb. The faster the crank is turned, the brighter the bulb shines.

A very rare type of star may be at the heart of it

If this is the case, it'd be a rather fantastic event. Typically, super luminous supernovae are the result of thermonuclear combustion — or a big star bomb, essentially. Those types of explosions shine bright, but they can't get nearly as bright as ASASSN-15lh. And when stars collapse rather than explode outward, they are usually a lot dimmer. If a collapsed star like a magnetar is fueling this phenomenon, it'd have to be spinning very fast and converting energy very efficiently to produce the shine that's being seen.

Or it's possible that something else is fueling the explosion altogether. For now, the magnetar model is the best explanation out of a very small pool of explanations. But the supernova is so unusual, it's hard to know exactly what its energy source is. "Can we even actually explain it with the magnetar?" asked study author Kris Stanek, an astronomer at Ohio State University, whose work on ASASSN-15lh was published in the journal Science. "Or if the magnetar model doesn’t work, and then we’d have to find some new way to explain this."

Two of the telescopes in use for the All Sky Automated Survey for SuperNovae (ASAS-SN) that discovered ASASSN-15lh. (Wayne Rosing)

Ironically, ASASSN-15lh could have easily been missed by astronomers. When searching for supernovae, scientists usually look through an approved list of galaxies. But the galaxy in which this supernova resides isn't in any catalogs. "It’s so far away that it’s actually very faint," said Stanek. "So it never made it to any lists and no one would have ever looked at it."

ASASSN-15lh could have easily been missed by astronomers

Fortunately, Stanek and his colleagues take an unbiased approach to finding supernovae. The team is part of the All Sky Automated Survey for Supernovae ASAS-SN, pronounced "assassin") collaboration, which uses a group of telescopes to scan the entire night sky every night. Using this method, the astronomers can observe stars in galaxies that aren't on the "map," which is how they spotted ASASSN-15lh. It kept showing up just above the team's detection threshold, prompting them to take a closer look.

After two weeks of observation, the astronomers figured out just how powerful their find was. The supernova was in a galaxy 3.8 billion light-years away and was 570 billion times brighter than our sun. It also packs more power than the most powerful supernova ever discovered by a factor of three.

While ASASSN-15lh may be intense, that doesn't mean it's big. "This is not precisely the 'largest' supernova, but the most luminous yet seen," said Robert Kirshner, a supernova expert at Harvard University, who was not involved in the study. In fact, the object at the center of the supernova itself is thought to be only 10 miles across.

Astronomers may have better answers soon enough

That's if the magnetar model is to be believed — and the magnetar model still raises other questions. In order to produce the brightness that is being seen, the magnetar would have to be spinning 1,000 times a second, and it would have to be converting energy to light with 100 percent efficiency. It's at the limit of what's even physically possible. That means if a magnetar is responsible, no supernova could be brighter than this one.

Still, the other explanations don't exactly satisfy either. The supernova could be fueled by some type of radioactivity, but it would need a staggeringly large source of decaying nickel for that (about 30 times the mass of the Sun). Plus, there's the possibility that this isn't even a supernova at all. It could be some unusual nuclear activity surrounding a supermassive black hole, though that would indicate a completely new event that's never been observed before.

Astronomers may have better answers soon enough. The Hubble Space Telescope will observe the star's host galaxy later this year, telling scientist more about the type of environment the supernova is in. "We’re looking for other clues — circumstantial evidence — about the nature of the galaxies where they form," said Kirshner. "This might tell us something useful about the chemistry of the star that exploded or its age." But no matter what is found, it's clear that ASASSN-15lh is shining way more light on how astronomers observe the cosmos.