Over time, it may grow larger as it swallows more gas and dust from nearby stars. But it would take many eons for it to become supermassive in size. Scientists estimate that it takes a black hole about 100 million years to double in size.

Previous X-ray data from the German-U.S. Roentgen Satellite and the Japanese-U.S. Advanced Satellite for Cosmology and Astrophysics suggested that such an object might exist in M 82.

That source was "astonishingly bright, about 100 times brighter than any black hole sources in our own galaxy," said Andrea Preswich of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts.

It also was "extremely variable," she said, increasing in brightness by a factor of seven over a period of just three months. Its intensity rose and fell every 600 seconds.

Because the object was not located at the center of the M 82 galaxy, the astronomers knew it could not be a typical supermassive black hole.

Rather, it fell into a class of black holes "somewhere between your compact economy cars and jumbo luxury SUVs," said Douglas Richstone, an astronomy professor at the University of Michigan, who is not connected with the Chandra team.

"I believe in the next five years, other teams working with Chandra will systematically investigate and discover a host of these objects," he said. "There's no physical reason why they can't exist."

In the past, our Milky Way Galaxy could have produced such mid-size black holes during periods of vigorous star formation. Hundreds could exist unseen, though it is doubtful that new ones might form today since there is little new star formation in the galaxy. New stars in the Milky Way only form at a rate of about one a century.

Astronomers instead will look for the mid-size black holes in more fertile places, like M 82, where a new star forms about once a decade.

"We'll find them in the bucket-loads, I suspect," Ward said. "This is like the tip of the iceberg. It would be amazing if M 82 is a special case."