U.S. astronomers have discovered a new size of the mysterious celestial objects called black holes. Now we know they come in medium as well as small and large sizes. This implies black holes are more common than previously thought.
Scientists know of nothing denser than black holes. They are so tightly packed and have such enormous gravity that even rays of light cannot escape.
Astronomers believe that all galaxies, like our Milky Way, have super-massive black holes at their center, 10,000 to 20,000 times the mass of our sun, acting as nature's vacuum cleaners by sucking everything near them inside. We also know of small black holes created when dying stars collapse inward.
Now, the Hubble space telescope has vindicated theorists by finding two of the intermediate variety, about 4,000 times the mass of our sun. Pennsylvania State University astronomer Stein Sigurdsson, who was not involved in the discovery, calls them the missing link between the small and very large black holes.
"What we didn't know was how you could get from one to the other or if they were completely unrelated," he said. "This seems to be the intermediate step, the stage that could take us from one to the other. Or possibly you form these intermediate mass black holes directly and then grow them up from there to the super-massive black holes."
Each of the two medium-sized black holes was discovered in unexpected places in our galaxy, by separate teams of researchers. An astronomer who detected one of them, Roeland Van Der Marel of the Baltimore institute that operates the Hubble, said they appeared in two collections of stars, called globular clusters, the oldest stars in any galaxy.
"Globular clusters were not expected or previously known to have black holes in their centers," he said. "Second of all, it now appears that black holes may be even more common in the universe than previously thought."
Like the super-massive black holes, the medium mass ones also have voracious appetites. The astronomers inferred their presence after noting that the speeds of stars in the clusters were faster than those in other clusters, indicating the holes' powerful pull.
Mr. Van Der Marel said these ancient star clusters may have had black holes when they formed in the earliest days of the universe. Add this conjecture to the fact that the mass of black holes is proportional to the mass of the galaxy or star clusters they inhabit and it is not hard to comprehend that the super-massive ones may be medium ones that grew, as galaxies grew around them. "These new results have important implications for understanding how galaxies, globular star clusters, and black holes formed in the early universe," he said.
Still unanswered, however, is which came first, the black holes or the star groupings? The director of Astronomy at the U.S. space agency NASA, Anne Kinney, calls this the classic chicken-and-egg question.
"Did the galaxy form and then the gravitational center of it suck the stars into it and form a super-massive black hole, or did the super-massive black hole form first and around it was formed these beautiful structures that we see everywhere in the night sky?" she asked. "Today's result, really, is a critical result for trying to understand this question."
Some astronomers like Marcia Rieke of the University of Arizona have a preference. She favors the notion that black holes preceded galaxy formation. "What's frustrating is that you can't tell for certain which model is right," she said. "But it certainly would be convenient if galaxies formed around black holes. It gives a starting center."
The discovery of intermediate-sized black holes in globular star clusters will prompt astronomers to look for more. The discoverers of the two in our Milky Way suggest that only one-fifth of clusters have them. But they disagree over whether the others never contained them or started out with black holes and lost them when the holes somehow floated away.