By Staff writer
updated 7/1/2009 2:55:20 PM ET 2009-07-01T18:55:20

Finally, a black hole Goldilocks would appreciate. Most black holes are either incredibly massive or amazingly compact and lightweight, but now scientists have discovered a medium-sized black hole that's just right.

The finding is the most solid evidence yet for a long-sought-after class of intermediate-sized black holes, researchers said. These middleweights, at about 500 times the mass of the sun, could represent a missing link between common stellar black holes, created by the death of a single star, and the supermassive variety that can pack the mass of millions or even billions of suns.

The discovery is an object on the outskirts of the ESO 243-49 galaxy, about 290 million light-years away. Astronomers detected a strong source of X-ray light, without a counterpart in optical light. These characteristics make the object much more likely to be a black hole than a foreground star or a background galaxy, researchers said.

"It's very difficult for us to say definitively 100 percent this is what it is," said study leader Sean Farrell, an astronomer at the University of Leicester in England. "But certainly it's the strongest candidate that we've seen so far."

Black holes are objects so dense that once captured, even light cannot escape their gravitational draw. Since they reflect no light, they are impossible to see directly. Astronomers can detect them through the strong radiation released by the whirlpooling disks of matter that fall into them, though.

In this particular case, the researchers think the X-rays they observed are being emitted as friction heats up gas and dust in the disk. The discovery, made with the European Space Agency¹s XMM-Newton X-ray space telescope, is detailed in the July 1 issue of the journal Nature.

The 10 most intriguing extrasolar planetsBlack holes in the smaller class form when a single massive star dies and gravity forces some of its matter to collapse into itself.

Scientists aren't sure about the origins of supermassive black holes, which populate the centers of many large galaxies. But one hypothesis suggests these behemoths are created when smaller black holes in dense environments, such as globular clusters, collide and merge.

"This is where our result comes into play in significance," Farrell told "If we can show that black holes between the two mass limits do exist then that gives us a natural progression of how something can go from stellar to supermassive in size."

These intermediate black holes are relatively rare because in most environments, such as a normal galaxy, stars aren't packed in tightly enough to cause frequent collisions, Farrell said.

The newly discovered intermediate black hole candidate was seen on the edge of a galaxy, where a globular cluster could be expected to reside. Because the galaxy is so far away, the cluster itself wouldn't be visible, but a middle-sized black hole's X-ray radiation could reach us.

Previous studies have found other medium-sized black hole candidates, but these are more ambiguous because they are fainter in X-ray light, Farrell said.

"There have been a number of theories to explain high [X-ray] luminosities without needing intermediate mass black holes," he said. "Where our paper stands out is that we've got a detection of an ultra-luminous source that is 10 times brighter than the previous record holder. It gets increasingly difficult to explain this higher luminosity."

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