Star cluster may harbor brown dwarfs.

For 50 years, researchers have searched unsuccessfully for brown dwarfs--dim, gaseous bodies that could represent some of the missing mass needed to keep the universe from expanding forever. Now a research team contends it has found six of the elusive objects in a nearby star cluster.

Too massive for a planet but too small to become a star, brown dwarfs represent a missing link between the two. To hunt for brown dwarfs, Claia Bryja of the University of Minnesota in Minneapolis and her colleagues scanned old photographic plates of the Hyades star cluster, a mere 150 light-years from Earth.

Several features of this Milky Way cluster enabled the team to discern two characteristics attributed to brown dwarfs, low luminosity and reddish color. First, the cluster's known distance from Earth made it easy to calculate the intrinsic brightness, and thus the mass, of candidate dwarfs within it. Second, the cluster's proximity to Earth allowed the team to spot dim bodies on the photographic plates.

The youth of the Hyades cluster--an estimated 600 million years old--also favored observations, Bryja says. Brown dwarfs radiate heat left over from their formation but have no other energy source, such as the nuclear furnace inside stars. A dwarf in the Hyades would still emit enough light for researchers to detect it, but it would have faded enough that it couldn't masquerade as a heavier, brighter object. Older brown dwarfs, lying farther from Earth, would "be cooled to oblivion" and could not be detected, notes Bryja.

Among 89 dwarf candidates found by Bryja's team, 12 were observed by the NASA Infrared Telescope Facility in Hawaii. Six of these had an infrared luminosity indicative of brown dwarfs.

The findings definitively identify the six candidates as brown dwarfs with masses about 70 times that of Jupiter and luminosities as low as one four-thousandth that of the sun, Bryja reported last January at a meeting of the American Astronomical Society in Atlanta. She and her team detail their work in an upcoming Astrophysical Journal Letters.