[Editors] MIT finds the universe’s faintest stars

[Teresa Herbert]

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MIT finds the universe’s faintest stars
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For Immediate Release
WEDNESDAY, DEC. 10, 2008

Contact: Teresa Herbert, MIT News Office
E: therbert at mit.edu, T: 617-258-5403


Whitney Clavin, Jet Propulsion Laboratory, NASA
E: whitney.clavin at jpl.nasa.gov, T: 818-354-4673

CAMBRIDGE, Mass. -- The two faintest star-like objects ever found, a  
pair of twin “brown dwarfs” each just a millionth as bright as the  
sun, have been spotted by a team led by MIT physicist Adam Burgasser.

“These brown dwarfs are the lowest power stellar light bulbs in the  
sky that we know of,” said Burgasser. And these extra-dim brown dwarfs  
may be the first discoveries of the predominant type in space. “In  
this regime [of faintness] we expect to find the bulk of the brown  
dwarfs that have formed over the lifetime of the galaxy,” he said. “So  
in that sense these objects are the first of these ‘most common’ brown  
dwarfs, which haven’t been found yet because they are simply really  
faint.”

Burgasser, an assistant professor of physics at MIT, said “both of  
these objects are the first to break the barrier of one millionth the  
total light-emitting power of the sun.” He is lead author of a paper  
about the discovery appearing in the Astrophysical Journal Letters on  
Dec. 10.

Astronomers had thought the pair of dim bulbs was just a single  
typical, faint brown dwarf with no record-smashing titles. But when  
Burgasser and his team used NASA’s Spitzer Space Telescope to observe  
the brown dwarf in infrared light, it was able to accurately measure  
the object’s extreme faintness and low temperature for the first time.  
The Spitzer data revealed that what seemed to be a single brown dwarf  
is in fact twins.

Brown dwarfs are compact balls of gas floating freely in space, too  
cool and lightweight to be stars but too warm and massive to be  
planets. The name “brown dwarf” comes from the fact that these small  
star-like bodies change color over time as they cool, and thus have no  
definitive color. In reality, most brown dwarfs would appear reddish  
if they could be seen with the naked eye.

When Burgasser and his collaborators used Spitzer’s ultrasensitive  
infrared vision to learn more about the object, thought to be a solo  
brown dwarf, the data revealed a warm atmospheric temperature of 565  
to 635 Kelvin (560 to 680 degrees Fahrenheit). While this is hundreds  
of degrees hotter than Jupiter, it’s still downright cold as far as  
stars go. In fact, the brown dwarfs, called 2MASS J09393548-2448279,  
or 2M 0939 for short, are among the coldest brown dwarfs measured so  
far.

To calculate the object’s brightness, the researchers had to first  
determine its distance from Earth. After three years of precise  
measurements with the Anglo-Australian Observatory in Australia, they  
concluded that 2M 0939 is the fifth closest known brown dwarf to us,  
17 light-years away toward the constellation Antlia. This distance  
together with Spitzer’s measurements told the astronomers the object  
was both cool and extremely dim.

But something was puzzling. The brightness of the object was twice  
what would be expected for a brown dwarf with its particular  
temperature. The solution? The object must have twice the surface  
area. In other words, it’s twins, with each body shining only half as  
bright, and each with a mass of 30 to 40 times that of Jupiter. Both  
bodies are one million times fainter than the sun in total light, and  
at least one billion times fainter in visible light alone.

Burgasser said studying these objects could help astronomers  
understand details of brown dwarf structure and evolution. These  
observations “allow us to see for the first time what the atmospheres  
of very old and/or very low mass brown dwarfs contain and how they are  
structured,” he said.

Other authors of this paper are Chris Tinney of the University of New  
South Wales, Australia; Michael C. Cushing of the University of  
Hawaii, Manoa; Didier Saumon of the Los Alamos National Laboratory,  
N.M.; Mark S. Marley, NASA Ames Research Center, Moffett Field,  
Calif.; and student Clara S. Bennett (’10) of MIT.

The work was funded in part by a NASA grant.

By David Chandler of the MIT News Office and Whitney Clavin of JPL

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