Spitzer seeks out Universe’s faintest stars
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: 15 December, 2008

NASA’s Spitzer space telescope has hunted down a pair of twin brown dwarf stars, each just a millionth as bright as the Sun – the faintest stars found yet.

Originally, astronomers thought the pair were in fact just one typical brown dwarf, but using Spitzer’s heat seeking infrared vision, it was able to resolve the object as two separate entities. "Both of these objects are the first to break the barrier of one millionth the total light-emitting power of the Sun," says Adam Burgasser of the Massachusetts Institute of Technology, and lead author of the paper about the discovery appearing in the Astrophysical Journal Letters. "These brown dwarfs are the lowest power stellar light bulbs in the sky that we know of. In this regime [of faintness] we expect to find the bulk of the brown dwarfs that have formed over the lifetime of the Galaxy. 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."

This artist's concept shows the dimmest 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. Image: NASA/JPL-Caltech.

Brown dwarfs are the misfits of the cosmos, and mark the line between planets and fully fledged stars. They are too warm and massive to be planets, but too cool and lightweight to be stars. Their feeble light output also means they are hard to find and the first brown dwarf wasn't discovered until 1995. Although hundreds more have been discovered since, astronomers say there are many more in space still waiting to be discovered.

The newly discovered dim duo of brown dwarfs is called 2MASS J09393548-2448279 (or 2M 0939 for short) after the Two Micron All-Sky Survey, 2MASS). But while the object was still believed to be a single star, astronomers used Spitzer to measure the atmospheric temperature of the body as 565 to 635 degrees Kelvin. While this is hundreds of degrees hotter than Jupiter, it is one of the coldest stars ever recorded. The researchers also calculated the pair’s distance as 17 light years from the Earth in the constellation Antlia.

But the brightness of the object was twice that expected for a brown dwarf with this particular temperature. The only solution was that the object must have twice the surface area, that is, two closely spaced but separate bodies, with each one 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. "These brown dwarfs are the lowest power stellar light bulbs in the sky that we know of," says Burgasser. "And like low-energy fluorescent light bulbs, they emit most of their light in a narrow range of wavelengths, in this case in the infrared."

According to the astronomers, there are even dimmer brown dwarfs scattered throughout the Universe, most too faint to see with current sky surveys. NASA's upcoming Wide-Field Infrared Survey Explorer mission will scan the entire sky at infrared wavelengths, and is expected to uncover hundreds of these inconspicuous characters. "The holy grail in the study of brown dwarfs is to find out how low you can go in terms of temperature, mass and brightness," says Davy Kirkpatrick, a co-author of the paper at NASA's Infrared Processing and Analysis Center at the California Institute of Technology. "This will tell us more about how brown dwarfs form and evolve."

Studying these objects could help astronomers understand details of brown dwarf structure and evolution, allowing astronomers 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.