Astronomers said today they've firmly identified two medium-mass black holes, supplying strong confirmation that these curious middleweights exist.

Black holes cannot be seen and so are detected by observing the environment around them. Scientists are convinced of the presence of so-called stellar black holes that result from the collapse of a single star, as well as galaxy-anchoring supermassive black holes weighing as much as billions of stars.

Two new studies, based on data from the Hubble Space Telescope, each found a middleweight in the unexpected environment of an ancient star cluster.

These globular clusters, as they are called, are thought to contain the oldest stars of the universe. They are densely packed but relatively quiescent places -- unlike the chaotic regions near the centers of galaxies that contain supermassive black holes. Researchers involved in the studies say globular clusters probably had black holes early in their histories and that they might fill a missing link in the understanding of how their far more massive cousins develop.

"The intermediate-mass black holes that have now been found with Hubble may be the building blocks of the supermassive black holes that dwell in the centers of most galaxies," said Karl Gebhardt of the University of Texas at Austin.

Gebhardt worked on a team led by Michael Rich of the University of California, Los Angeles, which found a black hole some 20,000 times the mass of our Sun in a globular cluster called G1. The cluster is 2.2 million light-years from Earth and is part of the Andromeda Galaxy.

Another team, led by Roeland Van Der Marel of the Space Telescope Science Institute (STScI) in Baltimore, detected a black hole about 4,000 times the Sun's mass. It sits in the center of a globular cluster called M15, roughly 32,000 light-years away and within our own Milky Way Galaxy.

The STScI manages Hubble for NASA.

"These findings may be telling us something very deep about the formation of star clusters and black holes in the early universe," Van Der Marel said. "Black holes are even more common in the universe than previously thought."

Theorists are still trying to understand how black holes form. One idea is that they develop all at once when a galaxy forms. Another method might be that a black hole "seed" is created and grows over time.

"The Hubble results add new credibility to the latter scenario," Van Der Marel said.

Globular clusters, being old, are like a snapshot of an earlier time. Because they have black holes and probably have had them since early on, they are now seen as good candidates for the "seeds" that theorists have been looking for, researchers from both teams agreed.

An interesting aspect to the discovery is that the newfound black holes pack the same percentage of mass relative to their host star clusters as do supermassive black holes in relation to their host galaxies. This fraction, about 0.5 percent, appears to suggest some underlying process for black hole formation and evolution, the researchers said.

Astronomers have hunted for black holes in globular clusters for nearly three decades, but ground-based visible-light observations can't resolve the central dense regions of stars well enough to see what all is there.

Hubble was used to examine the velocity of stars relative to each cluster as a whole, data that revealed the presumed black holes, explained Rutgers University researcher Carlton Pryor, who worked on the M15 discovery. Unlike a typical galaxy in which most stars are arranged in a single, rotating disk, the stars in a globular cluster are set up in a spherical pattern.

"It would look like a swarm of gnats," Pryor said in a telephone interview. "The motions are every which way." A large amount of mass hidden in the cluster's center is needed to explain the high speeds of the stars, Pryor said. While the central mass is presumed to be a black hole, researchers can't completely rule out other possible explanations, such as a collection of dense neutron stars.

The M15 observations were made three years ago and required painstaking analysis to reach the conclusions, announced at a NASA press conference today.

The presumed black hole in G1 was found using a similar technique. Because that cluster is so far away, however, researchers looked at the combined light of the cluster and inferred from it the velocities of stars.

The globular cluster M15 is visible in binoculars [use the map above-right]. G1 is a tougher target, requiring a good-sized amateur telescope.