Dim star wakes up

Artist's impression of the Chandra X-Ray Observatory

By BBC News Online science editor Dr David Whitehouse

Astronomers have detected an explosion on the surface of a failed star, showing that even some the dimmest objects in the Universe have their moments.

We really expected to see nothing - I hoped to see nothing

Professor Gibor Basri

The Chandra X-Ray Observatory detected the flare on a so-called brown dwarf - a body large enough to collapse and heat up, but not big enough to ignite the steady nuclear "fires" necessary to keep a star shining for billions of years.

Brown dwarfs will shine briefly and then cool down to become dead cinders drifting through space.

The flare, thought to be very similar to the flares seen on our Sun, is the first ever observed on a brown dwarf.

The scientists who saw it were engaged in a long observation to try to confirm that low-mass stars give off no X-rays. Instead, they noticed a bright light that flashed for two hours.

Free floating

"We really expected to see nothing - I hoped to see nothing, to prove that there was no hot atmosphere surrounding the brown dwarf," said Professor Gibor Basri, of the University of California, Berkeley.

Until 1995, brown dwarfs had not been detected with any certainty. Then Professor Basri confirmed the first brown dwarf using Hawaii's Keck 1 Telescope. Since then, several dozen have been found in nearby clusters or floating freely in the solar neighbourhood.

LP 944-20: Now you see it....

LP 944-20 is one of the nearest brown dwarfs, only 16 light years from Earth. It was first detected more than 25 years ago, but was thought at the time to be a very dim red star called a red dwarf.

Located in the constellation of Fornax, it is estimated to be about 500 million years old and has a mass that is at most 60 times that of Jupiter, or 6% of the Sun's mass. Its diameter is about one tenth that of the Sun, and has a rotation period of less than five hours.

Professor Basri and his collaborators planned the long observation by Chandra to eliminate the possibility that older brown dwarf stars have hot outer atmospheres or coronae.

The astronomers did not expect to detect any X-rays from LP 944-50 during their observations because the dwarf's atmosphere should be too cool. The fact that the X-ray satellite detected nothing for most of its 12-hour observation would seem to prove this hypothesis.

magnetic fields

"The flare was a bonus," Professor Basri said. "We've shown that older brown dwarfs don't have coronae, but the flare tells us they still have magnetic fields and also that subsurface flares occasionally punch through into the atmosphere."

...and now you do not

Eduardo Martin, of the California Institute of Technology, added: "This is the strongest evidence yet that brown dwarfs and possibly young giant planets have magnetic fields, and that a large amount of energy can be released in a flare."

The flare is believed to occur when magnetic fields twist and occasionally cross, arcing like an electrical short. When this happens, gas is explosively heated. The flares are thought to inject high-energy particles into the upper atmosphere of the star.

"This is an important confirmation of the trend that hot gas in the atmospheres of lower mass stars is produced only in flares," said Lars Bildsten, of the University of California.

Astronomers estimate that the energy emitted in the brown dwarf flare was comparable to a small solar flare, and was a billion times greater than observed X-ray flares from Jupiter.

Future observations of other brown dwarf stars are planned.