Failed star creates its own spotlight in the universe

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Evan Krape/University of Delaware

Although astronomers often refer to brown dwarfs as "failed stars," scientists at the University of Delaware have discovered that at least one of these dim celestial objects can emit powerful flashes of light. A research team led by John Gizis, professor in UD's Department of Physics and Astronomy, discovered an "ultracool" brown dwarf known as 2MASS 0335+23, with a temperature of only 4400°F that can generate flares stronger than the sun's. Gizis reported on the finding on June 13 at the annual meeting of the American Astronomical Society in San Diego.

"This brown dwarf is very young by star standards -- only 23 million years old," Gizis said. "It has lots of flares that are as hot as or hotter than the flares coming off full-fledged stars. This shows that the warmer brown dwarfs can generate flares from magnetic field energy just like stars. Our work shows, however, that colder brown dwarfs cannot generate flares even though they also have magnetic fields."

Brown dwarfs actually begin life just as stars do, from collapsing clouds of gas and dust, but they don't get big enough and hot enough for hydrogen and helium to fuse at their core, generating the nuclear reactions that keep a star burning bright for millions and billions of years.

Gizis and his team, including doctoral student Rishi Paudel, and collaborators from the University of California, San Diego and Harvard University, made the findings using NASA's Kepler space telescope, which monitored the brown dwarf every minute for three months.

Poring through thousands of images the size of a postage stamp, the team searched for spikes in brightness. Suddenly, the brown dwarf would get twice as bright for two to four minutes. This happened a dozen times over the three-month period.

"These flares are very powerful -- stronger than the sun's. They show what the sun could do when it was younger. It's like its acne is going away," Gizis said wryly of the sun, which is "middle-aged" now, at 4½ billion years old, 200 times older than this brown dwarf.

Gizis actually discovered the brown dwarf in 1999 when he was a postdoctoral fellow at the University of Massachusetts at Amherst working on NASA's 2MASS (Two Micron All Sky Survey) project. It is now known to be part of the Beta Pictoris moving group, an association of stars born at the same time and all moving in parallel in space some 63 light years away.

They were all originally part of an interstellar cloud, an amalgamation of dust, gas and space plasma. When this cloud collapsed, the brown dwarfs got scattered into space like the seeds of a dandelion in a puff of wind.

Gizis said he hopes to learn more about ordinary stars by studying the most unusual and extreme ones like brown dwarfs.

Among their most unique features, brown dwarfs do a complete spin every five hours -- now that's a very short day.

"In some respects, brown dwarfs are a lot like planets, especially Jupiter, the gas giant in our solar system," Gizis said. "They end up being a similar size because they are failed stars, and they get colder and colder with time like a planet does. They also have clouds on them. With Kepler, you can see what the clouds do for several months. You can see how much change occurs--that's the type of thing we're trying to figure out."

Gizis is looking for evidence of clouds, and for planets, too. The brightness dims when a planet comes in front of a brown dwarf or other star. Flares also can impact planets, as space weather watchers well know.

When the sun blasts out a massive X-class solar flare, releasing energy equivalent to a billion hydrogen bombs exploding at the same time, Earth can feel the effects, in damaged satellites and communications systems to electrical power grids.

"We think there are probably planets around brown dwarfs, so the flares generated by brown dwarfs could be a problem for them," Gizis said. "But as to whether such a planet might be a habitable one like Earth, Gizis thinks that's a long shot.

"It would be more like Mercury, which is pretty much fried," he said. "There's some debate about that. I guess we'll find out."

Source: University of Delaware