Astronomers get new spin on black holes.

They swallow everything in sight, warp space and time into an unfamiliar tangle, and trap even light in their clutches. Yet for something so complex, black holes are defined by just three properties: mass, charge, and spin.

By studying how ordinary stars are whipped around by their black hole companions, astronomers have measured the mass of many of these dense, invisible objects. Although they have not yet figured out a way to detect charge, scientists now report that they have indirectly determined the spin of several small black holes--those comparable in mass to stars--in the Milky Way.

"Now that we've learned how to measure a second property, spin rate, one might say that we are two-thirds of the way to understanding black holes," says study coauthor Shuang N. Zhang of the Universities Space Research Association at NASA's Marshall Space Flight Center in Huntsville, Ala.

The measurements reveal a specific relationship between the spin of a black hole and the energy it emits as gas is pulled from an orbiting companion onto a disk of material that surrounds and feeds the hole. Some black holes expel jets of material. Black holes with highly energetic jets--those that travel at nearly the speed of light--spin the fastest and in the same direction as the disk, Zhang and his collaborators report in the June 20 Astrophysical Journal Letters

The team, which includes Wei Cui of the Massachusetts Institute of Technology and Wan Chen of NASA's Goddard Space Flight Center in Greenbelt, Md., and the University of Maryland at College Park, identified two other indicators of spin. Black holes that emit their highest-intensity radiation at an X-ray energy of 1,000 electronvolts; were found to spin rapidly but in a direction opposite to the disk. Holes with X-ray energies that peak at one-fifth to one-tenth that amount spin slowly or not at all. This pattern fits all the stellar-mass black holes they've looked at, says Zhang, and may offer new clues about how these exotic beasts form and behave.

To deduce the spin, the astronomers took note of several related properties. A black hole consumes any material that gets closer than a certain distance. This distance, and the spectrum of radiation emitted by infalling material, depends on the black hole's spin.

Using data from four Earth-orbiting spacecraft, the team found the link between spin and X-ray emission. Two of the black holes they examined emit high-speed jets. Both rotate rapidly, and one of them, called GRO J1655-40, spins about 100,000 times per second.

When he heard Zhang present his results at a recent workshop, Mario Livio of the Space Telescope Science Institute in Baltimore got a jolt. In puzzling over the origin of a wide variety of jets in astronomy, Livio had predicted a year ago that black holes that spew such streams must spin rapidly. Unaware of that prediction, Zhang's team validated it. Calling the findings "intriguing but by no means certain," Livio says they provide a new step in deciphering the physics of black holes and jet production.