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Common star draws swift attention with unprecedented flare

May 20, 2008

College Park, Md. -- On April 25, one of our nearest stellar neighbors, a small, faint red dwarf known as EV Lacertae, unleashed the brightest flare ever detected from a normal star outside our solar system. The monster blast of radiation was picked up with NASA's Swift satellite, which scans space looking for Gamma-ray bursts coming from the edge of the universe.

"The sheer magnitude of this stellar flare is unprecedented, and it was produced in our own celestial back yard by a star of the most common type," said Rachel Osten, a Hubble Fellow at the University of Maryland and NASA's Goddard Space Flight Center in Greenbelt, Md.

Though EV Lacertae is only 16 light-years from Earth, it's not visible in the night sky. It is much cooler than the sun and shines with only one percent of the sun's light. The star's faint magnitude-10 glow is far below naked-eye visibility. However, the flare, which packed the power of thousands of solar flares, would have made it easy to see EV Lacertae had it occurred when the star was observably positioned in the night sky.

It's in the Spin

According to Osten, who studies nearby stars, EV Lacertae is young, with an estimated age of some 300 hundred million years, and small with a mass and diameter only about a third that of the sun. However, it rotates much faster than the sun, completing a rotation once every four days compared with every four weeks for the sun.

She and other scientists think this fast rotation combined with differences in the interior composition compared to that of the sun explain the presence of so many stellar spots on the star's surface, which indicate the generation of strong localized magnetic fields over more than half the surface of the star. The result is a star that is 100 times as magnetically powerful as the sun, and it is thought that the energy stored in and released by the interactions of these magnetic fields generate its powerful flares.,

"This star has a record of producing flares, but this one takes the cake," said Osten.

Flares release energy across the electromagnetic spectrum, but the extremely high temperatures produced in flares can only be studied with high-energy telescopes like those on Swift. Swift's wide field of view and rapid repointing capabilities are designed to study gamma-ray bursts, the most powerful explosions the Universe has seen since the Big Bang. However, these same capabilities make it ideal for also detecting and studying stellar flares. Two other X-ray observatories, Chandra and XMM-Newton, have studied EV Lacertae and other stars like it, but they have to be extremely lucky to catch and study powerful flares due to these observatories much smaller fields of view.

This flare's incredible brightness grabbed Swift's attention and enabled it to make detailed measurements. "This gives us a golden opportunity to study a stellar flare on a second-by-second basis to see how it evolved," said Stephen Drake of NASA Goddard.

Red Dwarfs, Killer Flares, and Earth-Like Planets

"Data like this on the flares of red dwarfs, also known as M stars, are important not only to help up understand the nature of these flares, but also because of renewed interest in searching for Earth-like planets around M stars," explained Osten.

About 75 percent of all stars in our Galaxy are M stars, which are long-lived, stable, and burn hydrogen. Until recently, M stars have been considered poor candidates for harboring habitable planets. This was, in part, because it was thought the violent flares generated by intense magnetic activity, could erode or even blast away planetary atmospheres. This problem was seemingly heightened by the fact that habitable zone for planets around a red dwarf would be much closer than that for larger, much more radiant stars like the sun.

However, recent theoretical studies have shown that the environment of M stars might not preclude their planets from harboring life have made M stars much more interesting to astronomers. "From a detection standpoint, M stars are ideal targets in the search for habitable planets, because the smaller size of these stars makes it much easier to detect smaller orbiting planets using transit and radial-velocity techniques," Osten said.

University of Maryland