Taking a census of brown dwarfs.

Using the Hubble Space Telescope, two teams have been in hot pursuit of some of the coolest objects in the universe.

Those objects, known as brown dwarfs, straddle a heavenly boundary: They're too massive to be planets yet too scrawny to be stars. Brown dwarfs are thought to arise the same way stars do--from the collapse of giant gas clouds.

They don't become stars, however, because they're not big enough to sustain fusion. Instead, they quickly fizzle and become hard to detect. To find some of the lowest-mass brown dwarfs known, both research groups relied on Hubble's near-infrared camera. The objects are too dim to be seen in visible light.

In one study, Joan R. Najita and Glenn P. Tiede of the National Optical Astronomy Observatory in Tucson, Ariz., along with John Carr of the Naval Research Laboratory in Washington, D.C., used the infrared instrument to examine brown dwarfs in a cluster of Milky Way stars known as IC 348. Because the cluster is extremely young, the brown dwarfs residing there are still somewhat bright, making it easier for Hubble's camera to pick them out.

In ferreting out brown dwarfs, the researchers had to distinguish these dim bodies from equally faint background stars. Because stars are much hotter, Najita's team could apply a kind of chemical thermometer to tell them apart. Water absorbs infrared light, but at the high temperatures in most stars, water molecules disassociate and can't block out the light. Brown dwarfs are cool enough to contain water in their atmosphere and absorb some infrared radiation.

In the Oct. 1 ASTROPHYSICAL JOURNAL, the researchers will report finding about 30 brown dwarfs in IC 348. Although dwarfs range in mass from about 10 to 80 times the mass of Jupiter, most of the ones the researchers found are on the lightweight end of the scale.

Although brown dwarfs are plentiful, they can't account for much of the unseen matter that astronomers believe resides in a giant halo around the Milky Way. In agreement with earlier studies, the researchers calculate that brown dwarfs can account for only 0.1 percent of the so-called dark matter in our galaxy's halo.

In the second Hubble study, to be reported in the Sept. 20 ASTROPHYSICAL JOURNAL, Kevin Luhman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and his colleagues used the infrared camera to image some 50 brown dwarfs in a nearby stellar nursery, the Orion nebula's Trapezium cluster. Their findings are similar to those of Najita's team.

The results provide an important confirmation of previous work, says Gibor S. Basri of the University of California, Berkeley. The Hubble data also offer the best census to date of brown dwarfs in clusters, he adds.