Nearby Star Erupts Dramatically

A flare-up on a relatively nearby star is perhaps the most powerful such event ever detected, astronomers said today.

Were such an event to occur on our Sun, it would trigger a mass extinction on Earth, the scientists said. Our Sun is not the sort of star capable of such a tantrum, however.

The flare was detected in December 2005 and announced today in a NASA statement. It occurred 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.

The star system is about 135 light-years from Earth.

"The flare was so powerful that, at first, we thought it was a star explosion," said Rachel Osten of the University of Maryland and NASA's Goddard Space Flight Center. "We know much about solar flares on the Sun, but these are samples from just one star. This II Pegasi event was our first opportunity to study details of another star's flaring as if it were as close as our Sun."

Solar flares on the sun originate in the corona, the outermost part of the Sun's atmosphere. The corona's temperature is about two million degrees Fahrenheit, while the Sun's surface, called the photosphere, is only about 6,000 degrees. The flare itself is a burst of radiation across much of the electromagnetic spectrum, from low-energy radio waves through high-energy X-rays.

The X-ray emission can last up to a few minutes on the Sun. On II Pegasi it lasted for several hours.

The flare involves a shower of electrons raining down from the corona onto the photosphere, heating the coronal gas to temperatures usually encountered only deep inside the Sun. Scientists think that the twisting and breaking of magnetic field lines lacing through the corona generate the particle acceleration and flaring.

The Sun likely erupted more violently in its youth, when it was spinning faster.

The two stars of are II Pegasi are not young. But they are only a few stellar radii apart. So tidal forces cause both stars to spin quickly, rotating in step once in 7 days compared to the sun's 28-day rotation period, the astronomers explained. Fast rotation is conducive to strong stellar flares.

"The tight binary orbit in II Pegasi acts as a fountain of youth, enabling older stars to spin and flare as strongly as young stars," said Steve Drake of NASA Goddard, a co-author with Osten on an upcoming paper to be published in the Astrophysical Journal.

The discovery was made with NASA's Swift satellite.