Globular star clusters like M22, located about 10,600 light-years away in the constellation Sagittarius, may start out with hundreds of black holes, but gravity soon slingshots all but one of them away.
Or so scientists thought.
Newly discovered radio waves coming from M22 indicate the massive cluster has at least two -- and possibly as many as 100 -- black holes, each about 10 to 20 times the mass of the sun.
"This suggests that while some black holes might be ejected from globular clusters, this process may not be as efficient as some have thought. Globular clusters might actually turn out to be good places to look for black holes, rather than poor ones," Michigan State University astronomer Jay Strader told Discovery News.
Black holes are objects so densely packed with matter that not even photons of light can escape their gravitational grip. They are found by studying their impact on orbiting partner stars and surrounding material.
Each of M22's black holes has a companion object that is being cannibalized -- not good for the objects, but fortuitous for astronomers who were able to pick up telltale radio signals of the phenomenon.
Analysis shows there may be many more stellar-mass black holes, perhaps as many as 100, lurking inside the cluster, a spherical collection of stars orbiting around the galactic center.
"That many black holes will have an effect on the structure and evolution of M22," noted astronomer Stefan Umbreit with Northwestern University.
The black holes likely would keep the cluster's core from contracting and slow the evolution of the cluster as a whole.
M22's black holes are estimated to be about 1.6 light years from one another. Light travels at about 186,000 miles per second.
Strader and his colleagues actually were hunting for larger black holes, those with 100- to 1,000 times the mass of the sun, when they stumbled upon M22's pair.
"The stellar-mass black holes we found were a surprise," Strader wrote in an email.
The research is published in this week's Nature.