One billion miles per gallon -- that's how far a car could travel if it were powered by a typical black hole. Scientists with NASA's Chandra X-ray Observatory recently arrived at this estimate after determining black holes are the most fuel-efficient "engines" in the universe, a discovery that highlights a black hole's economical performance and its benefits.

Image to right: Chandra scientists wondered why twin bubbles developed in vast clouds of hot gas surrounding some supermassive black holes. Credits: X-ray: NASA/CXC/Stanford U./S. Allen et al. IR: NASA/ESA/McMaster Univ./W. Harris Radio: NRAO/VLA + View Larger Image

"Just as with cars, it's critical to know the fuel efficiency of black holes," said Stanford University's Steve Allen, lead author of the study. "Without this information, we cannot figure out what is going on under the hood, so to speak, or what the engine can do."

Observations made with Chandra show that as matter falls toward a supermassive black hole, it releases enormous amounts of energy. The surprise revelation is that most of this energy goes into producing immense jets of particles that erupt from it at nearly the speed of light.

The jets were discovered when images from Chandra showed something was blowing giant bubbles into gas clouds surrounding black holes. Scientists were able to estimate the strength of the jets by determining the force and size needed to form such cavities. Their intensity shows researchers that black holes make mind-bogglingly good use of available resources.

"If a car was as fuel-efficient as these black holes, it could theoretically travel over a billion miles on a gallon of gas," said co-author Christopher Reynolds of the University of Maryland.

Image to left: An artist's illustration shows a close-up view of a supermassive black hole in a galaxy's center. Gas becomes hotter as it approaches the black hole, turning from red to yellow to white. Most of the gas is swallowed by the black hole, but some is launched in jets away from the black hole at almost the speed of light. Credit: CXC/M. Weiss + View Larger Image

Engines, of course, perform work. In the case of black holes, the results of this study suggest the job of their jets is to possibly limit the numbers of stars that form near them.

Untold numbers of stars could condense from a black hole's hot gas clouds if the clouds are allowed to cool enough. The unbridled process could sprout billions of extra stars, perhaps crowding the cosmic neighborhood. The high-velocity jets stop this from happening by keeping cloud temperatures high and star formation in check.