Mystery of the Brown Dwarf

by David F. Salisbury 2,106 views No Comment Print  E-mail

Keivan Stassun

Pity the brown dwarf. It’s too large to be a planet, but too small to be a star.

Brown dwarfs are smaller and dimmer than true stars. Only in recent years have improvements in telescope technology allowed astronomers to catalog hundreds of faint objects that may be brown dwarfs. Yet to actually determine if a faint object is a brown dwarf, scientists needed a way to estimate their masses, because mass distinguishes stars and starlike objects. 

The discovery of an eclipsing pair of brown dwarfs in the Orion Nebula by a team of astronomers led by Keivan Stassun, assistant professor of astronomy at Vanderbilt, provides the first direct measurement of the mass, size and surface temperature of a brown dwarf. Astronomers can now compare the information on the pair to other possible brown dwarfs.  

While surveying the Orion Nebula, Stassun and his colleagues, University of Wisconsin professor Robert Mathieu and Space Telescope Science Institute astronomer Jeff Valenti, found the brown dwarfs orbiting each other around an axis perpendicular to the line of sight to Earth.

Measuring a Dwarf

Because of their special orientation, the two objects periodically eclipse each other. These eclipses cause regular dips in the brightness of the light coming from their joint image. By precisely timing these occultations, the astronomers determined the orbits of the two objects. This information, along with Newton’s laws of motion, allowed the team to calculate the mass of the two dwarfs.

The astronomers also calculated the size of the dwarfs by measuring the width of the dips in their light curve. By measuring variations in the light spectrum coming from the pair, the astronomers also determined their surface temperatures.

“This binary pair is a Rosetta stone that will help unlock many of the mysteries regarding brown dwarfs,” Stassun says. “We understand how stars form in the crudest sense. But many of the details of the process remain a mystery, particularly the factors that determine what a star will weigh.”

Photo by Daniel Dubois.