Energy-Saving Space Engines: Black holes can be green

12:32pm, April 26, 2006

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.

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