Rogue Black Holes Roam Milky Way

Jan. 9, 2008 -- Hundreds of rogue black holes may be roaming around the Milky Way waiting to engulf stars and planets that cross their path, U.S. astronomers said Wednesday.

The astronomers believe these "intermediate mass" black holes are invisible except in rare circumstances and have been spawned by mergers of black holes within globular clusters -- swarms of stars held together by their mutual gravity.

These black holes are unlikely to pose a threat to Earth, but may engulf nebulae, stars and planets that stray into their paths, the researchers said.

"These rogue black holes are extremely unlikely to do any damage to us in the lifetime of the universe," said Kelly Holley-Bockelmann, an assistant professor of physics and astronomy at Vanderbilt University in Nashville, Tenn.

"Their danger zone, the Schwarzschild radius, (or gravitational radius) is really tiny, only a few hundred miles. There are far more dangerous things in our neighborhood."

The evidence for "intermediate mass" black holes, as opposed to supermassive or stellar-mass black holes, is still largely theoretical and therefore controversial. Only two tentative observations of objects of this sort have been made to date.

But the theoretical case for them is strong, and X-ray observations in recent years suggestive of their existence encouraged Holley-Bockelmann and colleagues at the University of Michigan and Penn State University to simulate what would happen if intermediate mass black holes combined with stellar-sized black holes, which are plentiful in globular clusters.

Using sophisticated computer modeling, they calculated that these mergers would generate hundreds of mid-size black holes, and that the force of their combinations would catapult them out of the globular cluster at speeds of up to 2,485 miles per second.