By Paul Recr
AP Science Writer
Tuesday, September 17, 2002; 7:40 PM[/size]
WASHINGTON –– Astronomers say they have found a new type of black hole and now believe those mysterious celestial objects exist in a variety of sizes, from small to supermassive.
Two teams of astronomers, using the Hubble Space Telescope and other instruments, have found evidence for a type of medium-sized black hole, a class of the objects that has never before been seen.
At a NASA news conference on Tuesday, Roeland Van Der Marel of the Space Telescope Science Institute in Baltimore said his group has confirmed the existence of a mid-sized black hole in a cluster of stars called M15, which is in the constellation Pegasus, about 32,000 light years away.
Van Der Marel said the black hole has about 4,000 times the mass of the sun, which makes it the smallest galactic black hole ever found.
Another group, led by Michael Rich of the University of California, Los Angeles, looked into a cluster of stars called G1 and found a black hole that has about 20,000 times the mass of the sun. G1 is in the Andromeda galaxy, some 2.2 million light years away.
It's long been known that the centers of most galaxies contain what is called a supermassive black hole, equal in mass to millions of suns. Astronomers also have found stellar black holes which form from the collapse of a star five to 10 times the mass of the sun.
The discovery fills in a black hole blank that has bothered astronomers for years, said Steinn Sigurdsson, who attended the news conference as an independent commentator.
"We have had this big desert in black holes where there was nothing in between one with a few solar masses and the supermassive black holes found at the center of galaxies," said Sigurdsson. "Now we're filling in that void."
Neither G1 nor M15 is a true galaxy. Each contains perhaps only a million stars, far fewer than billions of stars in galaxies such as the Milky Way.
G1 and M15 are known as globular clusters, a type of star swarm often found in orbit of bigger galaxies. Globular clusters are thought to contain some of the oldest stars in the universe. The new black holes are the first to be found within globular clusters.
Using the motion of stars within the clusters, the astronomers could determine the mass of the star groups and of the black holes. They found that the ratio of the mass of the black holes to the mass of the globular clusters was precisely the same as the ratio between supermassive black holes and galaxies.
This suggests a neat, uniform universal truth that applies the same mass ratio to black holes of all sizes everywhere, said Karl Gebhardt of the University of Texas, a member of the G1 observing team.
"Nature is usually messy," he said. "Almost never do we have a situation where nature is perfect, but this may be one."
The new discovery may help astronomers answer a basic question: Which came first, galaxies or black holes? Both theories have been proposed and finding intermediate-sized black holes may help answer that question, said Sigurdsson.
–––
On the Lin:
Space Telescope Science Institute: http://oposite.stsci.edu/
© 2002 The Associated Press[/size]
.
@ Unc!