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Star gives off superflare equal to 80 billion megatonnes of TNT. That’s a lot

Artist’s impression of a superflare on an L-dwarf. University of Warwick/Mark Garlick

It’s small but it’s mighty: A tiny star the size of Jupiter has been observed giving off a stellar flare of truly epic proportions. The massive superflare was 10 times more powerful than any flare from our Sun and is raising questions about how much energy small stars can hold.

The star is called ULAS J224940.13-011236.9 and is of a type called an L dwarf star. It is located 250 light-years from Earth, and it was the brightness of the flare that allowed astronomers to locate this star at all.

“We knew from other surveys that this kind of star was there and we knew from previous work that these kinds of stars can show incredible flares,” lead author James Jackman, a Ph.D. student in the University of Warwick’s Department of Physics, said in a statement. “However, the quiescent star was too faint for our telescopes to see normally — we wouldn’t receive enough light for the star to appear above the background from the sky. Only when it flared did it become bright enough for us to detect it with our telescopes.”

The flare happened on August 13, 2017 and was one of the biggest flares ever seen on an L dwarf star. The flare was truly colossal, giving off the equivalent energy of 80 billion megatonnes (or 80 gigatonnes) of TNT and making the star appear 10,000 times brighter than usual.

L dwarf stars are tiny by star standards, with only just enough mass to be considered a star and not a brown dwarf. Any smaller and the L dwarfs would not be able to fuse hydrogen into helium, the primary feature of a star. This means the L dwarfs are relatively cool and emit radiation mostly in the infrared spectrum.

The fact that a small, relatively weak star could produce such a massive flare was surprising: “It is amazing that such a puny star can produce such a powerful explosion,” Jackman’s Ph.D. supervisor Professor Peter Wheatley said. “This discovery is going to force us to think again about how small stars can store energy in magnetic fields. We are now searching giant flares from other tiny stars and push the limits on our understanding of stellar activity.”

The findings are published in the journal Monthly Notices of the Royal Astronomical Society: Letters.

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