Only two sizes of black holes have ever been spotted: small and super-massive. Scientists have long speculated that an intermediate version must exist, but they've never been able to find one until now.

Astrophysicists identified what appears to be the first-ever medium-sized black hole, pictured in an artist's rendition above, with a mass at least 500 times that of our Sun. Researchers from the Centre d'Etude Spatiale des Rayonnements in France detected the middling hole in a galaxy about 290 million light-years from Earth.

The discovery may shed some light on the origins of super-sized black holes like the one at the center of our own galaxy. These astral heavyweights top out at several million to several billion times the mass of the Sun, but their origin remains a mystery.

Small black holes, between three and 20 times the mass of the sun, are created when big stars collapse and leave behind a gravitational pull strong enough to block nearby light rays. Researchers have speculated that super-massive black holes result from the successive fusion of many smaller black holes. But without finding evidence of a medium-size hole, it was a tough theory to prove.

"The existence of such intermediate-mass black holes is in dispute," the French scientists wrote Wednesday in Nature, "and though many candidates have been proposed, none are widely accepted as definitive."

The new discovery is the most convincing evidence to date that medium black holes exist. Using the European Space Agency's XMM-Newton X-ray space telescope, the researchers identified a radiation source that gives off X-rays 260 million times brighter than the radiation of the Sun.

Called "Hyper-Luminous X-ray Source 1," the structure sits on the edge of galaxy ESO 243-49. Because of the source's physical characteristics and the pattern of its radiation, the researchers conclude it must be a black hole more than 500 times the mass of the sun: not too big, not too small, and the first of its kind.

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Image: Heidi Sagerud