Astronomers have positively identified two types of black holes: the very big, with a mass of millions to billions times that of the sun, and--at least in astronomical terms--the very little, with a mass of about two to 10 suns. But some recent findings point to the existence of intermediate-size black holes equivalent to 100 to 10,000 stellar masses. New research published today in the journal Nature provides insight into how such holes might form.

Stars within a typical galaxy are so far apart that the chances of two colliding are slim to none. But in some dense regions, known as star clusters, they are much more likely to cross paths. Scientists led by Simon F. Portegies Zwart of the University of Amsterdam developed a powerful computer simulation to track the behavior of stars in two clusters located in galaxy M 82 over the course of millions of years. The model reveals that stars with the largest mass naturally sink toward the center of a cluster where they are even more likely to careen into other celestial bodies. The merging of two colliding bodies creates a single entity with a larger mass. And because the stars size and gravitational force increase as its mass expands, so, too, does its chance of further collisions. The effect thus snowballs into what the team calls "runaway growth." Whether or not such a growing star will become an intermediate black hole depends on how fast its mass expands--or how many collisions it manages to make--before it explodes into a supernova, the researchers report.

The notion of runaway star growth not only suggests a mechanism to explain the formation of intermediate black holes but also offers additional hints to the origins of supermassive holes, a topic that has long been debated. According to one theory, these giants lump together from smaller black holes, but how something so big could arise from parts so small has until now remained a mystery. In an accompanying commentary astrophysicist Nate McCrady of the University of California at Berkeley calls the new modeling results "tantalizing." He notes: "This could well be how the building blocks of supermassive black holes formed."