The normal activity on our Sun is puny in comparison

Our Sun also flares when twisted magnetic field lines in the solar atmosphere suddenly snap - but this was on a far bigger scale, scientists say.

The energy released by the II Pegasi star was equivalent to about 50 million trillion atomic bombs, they calculate.

If the Sun was ever to produce such an outburst, it would blast the Earth with radiation and almost certainly cause a mass extinction.

Fortunately, there is no trace in the geologic record that our Sun has ever let go in this fashion, and Earth's magnetic field does a robust job of deflecting most of the high-energy particles and radiation our star will hurl at the planet in a normal flare.

Nonetheless, researchers are keen to learn more about the type of "super flares" seen by Swift, and the orbiting observatory's ability to turn its instruments rapidly on interesting cosmic events has revealed new data about the emission of X-rays in the earliest stages of this colossal phenomenon.

Rachel Osten of University of Maryland and the US space agency's Goddard Space Flight Center has been presenting details of her team's Swift study to the Cool Stars 14 meeting in Pasadena, California.

"The flare was so powerful that, at first, we thought it was a star explosion," she said.

"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."

Lobster survey

Scientists in Europe are hopeful of launching an X-ray telescope early in the next decade that will survey the sky for super flares as part of its mission.

"We simply don't have good statistical data on these flares; so one of the things Lobster is designed to do is survey the whole sky for these big flashes of light," said Dr Paul O'Brien, a scientist working on both the Lobster and Swift projects at the UK's University of Leicester.

"It has been suggested that such events would sterilise planets and so knowing how often they occur would put limits on how many civilisations might be out there. Presumably, our own Sun doesn't do it, otherwise we wouldn't be here."

Dr O'Brien said the latest study underlined again Swift's remarkable versatility.

The observatory was launched to hunt down gamma-ray bursts, the extremely intense but fleeting flashes of even higher energy light thought to signal the cataclysmic collapse or giant stars.

"Swift has the ability to turn around very quickly and look at something else on the way to studying another gamma-ray burst, and it's not a waste of observing time because it only took a minute to get there," the Leicester scientist said.

Swift was launched to observe gamma-ray bursts

II Pegasi is a two-star, or binary, system that is 135 light-years from Earth in the constellation Pegasus.

The star that produced the giant flare is 0.8 times the mass of the Sun; its companion is 0.4 solar masses.

The stars are close, only a few stellar radii apart. As a result, tidal forces cause both stars to spin quickly, rotating in step once in seven days compared to the Sun's 28-day rotation period. Fast rotation is thought to be conducive to strong stellar flares.