In recent years, however, researchers have tracked down a third type of black hole by making X-ray observations of distant galaxies. Neither stellar-sized nor supermassive, these missing links are something in between. A pair of recent studies suggests two ways they may have formed: one as a result of feeding off of a nearby star, the other from the runaway collisions of stars in a dense star cluster.

"I think that were not quite there yet where we can say we know these are intermediate black holes for sure," said University of Michigan astronomer Joel Bregman. "But one of the first steps in trying to figure them out is to find out whats going on around them."

Bregman's team discovered visible stars orbiting objects thought to be midsize black holes. He and his colleagues used overlapping images of extremely bright X-ray objects, thought to be midsize black holes, taken by the space-based Chandra X-ray Observatory and visible-light pictures from the Hubble Space Telescope. They found young stars locked in binary systems, apparently feeding gas to midsized black holes as they orbited each other.

"These are remarkable and unexpected objects, and challenge anyone who thinks about how stars evolve," Bregman said of midsize black holes during a telephone interview. "Their formation is not predicted by the evolution of normal massive stars."

The genesis of these midsize black holes is just one of the mysteries surrounding them. Their masses can measure up to a 1,000 times that of the Sun, making them much larger than stellar black holes and therefore not the products of collapsed single stars. But at the same time, theyre nowhere near the size or intensity of supermassive black holes. Learning how they formed could shed new light on stellar evolution, as well as the structure of the galaxy.

If nothing else, Bergmans study suggests that the black holes observed are associated with young massive stars, a notion supported by the computer-modeling work of astrophysicists Steve McMillan of Drexel University and Simon Portegies Zwart of the University of Amsterdam.

Their research found a potential source of midsize black holes in dense, young star clusters, such as the Arches located near the center of the Milky Way. The Arches cluster consists of tens of thousands of stars pack tightly together, all just 100 light years from the galactic center, or about 30,000 light years from Earth.

In computer simulations one star could collide with a neighbor, then another, setting off a runaway growth pattern over a period of a million years. The collection of stars could eventually command enough mass to form a midsize black hole and slowly sink toward the galactic center.

"Its very much like a snowball running down a hill," McMillan told SPACE.com. "The idea of star clusters leading to the formation of intermediate black holes is interesting because it provides a mechanism for black holes to reach the center of the galaxy."

Scientists have been perplexed in attempts to determine exactly where supermassive black holes came from, and its possible, though unproven, that at least some of them resulted from the merging of midsize black holes, he added.

McMillan said there is still more to learn about intermediate black holes and their role in the universe, and his computer-modeling studies are "just another weapon in our tent for understanding these objects."

The findings of both studies were presented at a meeting of the American Astronomical Society in January.

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