Scientists using NASA’s Swift satellite have spotted a stellar flare on a nearby star so powerful that, had it been from our sun, it would have triggered a mass extinction on Earth. The flare was perhaps the most energetic magnetic stellar explosion ever detected. The flare was seen in December 2005 on a star slightly less massive than the sun, in a two-star system called II Pegasi in the constellation Pegasus. It was about a hundred million times more energetic than the sun’s typical solar flare, releasing energy equivalent to about 50 million trillion atomic bombs. Fortunately, our sun is now a stable star that doesn’t produce such powerful flares. And II Pegasi is at a safe distance of about 135 light-years from Earth.
https://www.nasa.gov/mission_pages/swift/bursts/monster_flare.html