Black holes are the most fuel-efficient engines in the universe, the US space agency NASA said.
NASA scientists said Monday that the new findings, using the agency's Chandra X-Ray Observatory, gave insight on how nine black holes generated their huge energy in space.
If a car were as fuel-efficient as these black holes, it would travel more than 1 billion miles (1.6 billion km) on one gallon (3.8 litres) of petrol, said Christopher Reynolds of the University of Maryland.
Black holes - former stars so massive that even light cannot escape them - also are 20 times more efficient than nuclear reactors, the study found.
The scientists said they calculated the size of the galaxies' inner regions to estimate how much "fuel" - gas in space that is drawn into the black holes - is available.
Then, the observatory's images were used to estimate how much power was developed by the resulting jets of high-energy particles to create huge bubbles, or cavities, in the galaxies' hot gas.