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 last week, 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.

Filling in a void

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.