Dim discovery: This artist's concept shows the faintest star-like bodies currently known – twin brown dwarfs referred to as 2M 0939. The twins, which are about the same size, are drawn as if they were viewed close to one of the bodies.
Credit: NASA/JPL-Caltech
SYDNEY: The faintest star-like objects ever found, a binary pair of brown dwarfs each just a millionth as bright as the Sun, have been spotted by physicists in the USA.
"These brown dwarfs are the lowest power stellar light bulbs in the sky that we know of," said Adam Burgasser lead researcher behind the discovery at the Massachusetts Institute of Technology in Boston – and they may also be one of the most common types of star.
"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."
Between a star and a planet
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 comes from the fact that these small star-like bodies change colour over time as they cool, and thus have no definitive colour. In reality, most brown dwarfs would appear reddish if they could be seen with the naked eye.
Burgasser and his team used NASA's Spitzer Space Telescope to observe, what was thought to be a single brown dwarf in infrared light, which revealed it was in fact a pair of vanishingly faint, cool stars. Their research is detailed this week in the Astrophysical Journal Letters.
At just a few hundred degrees hotter than Jupiter – somewhere between 290ºC and 360ºC – the brown dwarfs, collectively 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 Epping, Australia, they concluded that 2M 0939 is the fifth closest known brown dwarf to us, 17 light-years away toward the constellation Antlia.
Puzzling data
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.
With MIT and NASA's JPL.
