Scientists: There are also medium-sized black holes
Kenneth Chang, New York Times
The X-ray photograph of the discovered black hole (the bright spot in the center) as taken by NASA
Astronomers announced yesterday that they have convincing evidence for the existence of a new type of black hole - larger than the black holes formed by collapsed stars, but much smaller than the giant holes found at the center of galaxies.
Until today, scientists believed that black holes - objects that collapsed in on themselves so that even light cannot escape their gravity - could be stars that collapsed in on themselves and their mass is at most 10 times that of the Sun; Or the enormous black holes that are apparently at the center of every galaxy, whose mass can reach that of a billion suns. The newly discovered black hole, as in the legend of Goldilocks and the Three Bears, is between these two sizes: its mass is at least about 500 suns.
After astrophysicists identified the existence of medium-sized black holes, they faced a new puzzle: to explain how the universe creates such black holes. "It is impossible for such an object to have formed from the collapse of a single star, because it is too large," says Dr. Philip Cart, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
The astronomers estimated the size of the black hole by measuring X-rays, which are emitted when matter falls into an abyss. A team of American and Japanese astronomers working at NASA's Chandra Observatory detected a bright X-ray beam in the galaxy M82, about 12 million light-years from Earth. They noticed that the brightness of the beam changed by about 10% every 10 minutes. Apparently , oscillations such as these are created by vibrations in the ring of gas and dust swirling around black holes.
"It is similar to a bell," explains Dr. Cart. As low-frequency rings are produced by large bells, the slower fluctuations in the X-rays, and the degree of brightness of the rays, indicate a larger black hole, between a few hundred suns and 100.
Already in the 82s, Dr. Fabiano Giuseppina from the Harvard-Smithsonian Center observed sources of bright X-rays in distant galaxies. Giuseppina suggested that the source of the rays might be medium-sized black holes. Last year, two astrophysicists from Carnegie Mellon University reported observations, which they said Attest to the existence of medium-sized black holes in MXNUMX Astronomers from NASA's Goddard Institute for Space Studies have announced that they have spotted three medium-sized black holes in other galaxies.
But all these claims were based on data from outdated satellites, which could not locate the exact point from which the X-rays came, or distinguish between a medium black hole and a group of small black holes. The current and precise observations from Chandra identified the location of the black hole - 600 light years from the center of the galaxy.
"If it was a black hole with the size of a million solar masses, I think it would sink into the center of the galaxy, like a stone in honey," says Dr. Douglas Richstone, a professor of astronomy at the University of Michigan. In his opinion, in this case it is clearly a medium-sized black hole .
The black hole in galaxy M82, located inside a dense star cluster, may have grown by swallowing stars and small black holes. Another possibility is that a large cloud of gas collapsed straight into a black hole.
Now, astronomers hope that by understanding how medium-sized black holes form, it will be possible to better understand the origin of supermassive black holes - another mystery in itself.
{Appeared in Haaretz newspaper, 14/9/2000}
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