A small red dwarf star has erupted with the brightest flare ever seen from a normal star other than the Sun. If the star hosted planets, the flare could have killed any life that might have existed on the unlucky worlds.
The flare, which lasted 8 hours, was first detected by NASA's Wind satellite on the morning of 25 April. The agency's Swift satellite detected it less than two minutes later, and traced it to a red dwarf star called EV Lacertae, which is just a third as massive as the Sun and puts out just 1% or so of its light.
Normally, EV Lacertae, which lies 16 light years from Earth, is far too dim to see without a telescope.
But the flare briefly outshone the star itself, making it temporarily bright enough to have been seen with the naked eye. The flare also temporarily made EV Lacertae one of the brightest sources of X-rays in the sky.
Some flares from dense stellar corpses called neutron stars have appeared brighter in X-rays from Earth, but this is the brightest flare ever observed from a normal star.
Scientists believe such flares of radiation and charged particles are triggered when a star's twisted magnetic field lines suddenly snap into a smoother configuration. This releases huge amounts of energy, which heats up material in the star's atmosphere and fires charged particles out into the surrounding space.
Red dwarfs such as EV Lacertae generally boast more flare activity than larger stars because their insides churn more violently. It is also just a few hundred million years old - younger stars spin faster, creating more of the magnetic activity that can trigger flares.
The 5-billion-year-old Sun likely sent out flares as powerful as EV Lacertae's when it was younger, and sometimes emits flares today that appear brighter, simply because it is so close to the Earth.
The flare would have been dangerous for any life-bearing planets around EV Lacertae. It would have destroyed the protective ozone layer of any such planet, allowing dangerous amounts of the star's ultraviolet light to bathe the planet's surface.
Because the star is so dim, a planet would have to orbit it at a mere 1/10 of the Earth-Sun distance in order for liquid water - thought to be a requirement for life - to be present on its surface. That proximity would make the flare all the more deadly to any life, says Rachel Osten of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US, who led the Swift observations.
"We think that these really powerful flares would have an adverse effect on the ability of life to develop," Osten told New Scientist.
Other, more intrinsically powerful flares have been observed, such as the II Pegasi outburst of 2006, but the EV Lacertae flare appeared brighter as seen from Earth because of the star's proximity to Earth.
Because of its brightness, astronomers have been able to observe this flare in greater detail than any previously, aside from those occurring on the Sun.
"This means that we can study the way the [light] spectrum changes with time and hopefully deduce new things about the physics of flares," Stephen Drake of Goddard told New Scientist. "Many questions [still] remain unanswered about their underlying origin."
The flare was so bright that it overwhelmed Swift's ultraviolet and optical telescope, triggering it to temporarily shut down to avoid damage. But the satellite's X-ray telescope was able to continue watching the flare to the end.
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