Energy-saving space engines: black holes can be green.

Some seemingly quiet black holes are actually efficient engines that emit jets of high-energy particles. This finding, from the first study to directly measure the efficiency of black holes, offers a hint as to why the universe isn't more crowded with stars.

All black holes swallow matter and spit out energy. Their gravitational pull traps clouds of hot, X-ray-emitting gas, and the black holes spew radiation or jets of high-energy particles.

The energy that black holes send out affects their environments. Scientists had presumed that young black holes producing quasars, which are beacons of light, are highly efficient. However, that efficiency hasn't been directly measured because the quasars are too bright.

Instead of focusing on quasars, a team of scientists led by astrophysicist Steven W. Allen of Stanford University used NASA's Chandra X-Ray Observatory to look at nine supermassive black holes, which are older than quasars and lie at the centers of nearby giant elliptical galaxies. "These are the boring old black holes that we thought had stopped doing anything interesting a long time ago," says team member astrophysicist Christopher S. Reynolds of the University of Maryland at College Park. The team's findings, announced this week, will be published in an upcoming Monthly Notices of the Royal Astronomical Society.

Though these black holes produce relatively little radiation, previous Chandra observations had noted the formation of large cavities in the surrounding gas clouds, as if the black holes were blowing bubbles tens of thousands of light-years across.

The team calculated the amount of energy needed to form those bubbles and compared it with the growth of the gas disks that encircle the black holes. From those results, the scientists estimated that each black hole converts about 2.5 percent of the mass of captured gas into jets of particles.

That's about 25 times as efficient as nuclear power, Allen says.

"If you could make a car engine as efficient as a black hole engine, you could get about a billion miles out of 1 gallon of gas," he says. "That's green by anyone's book," The finding suggests that not just quasars but all black holes are efficient, whether they expel energy as radiation or jets of partides, Reynolds adds.

This research represents the "next big step" in understanding what black holes do and how galaxies and clusters evolve, says astrophysicist Kim Weaver of NASPA's Goddard Space Flight Center in Greenbelt, Md., who was not on the study team.

The findings may hold clues to the puzzle of why galaxies aren't as big a cosmologists' models predict. Moving only slightly slower than the speed of light, the jets from black holes slam into the surrounding gas and heat it, preventing it from cooling enough to form stars, Weaver says.

These black holes "may be preventing galactic sprawl from taking over the neighborhood," she adds.