Astronomers have concluded that massive black holes are at the centers of two of the Milky Way's neighboring galaxies -- findings that indicate the galaxies' masses could force the eventual collapse of these star systems.

The researchers said their conclusions, announced yesterday by the National Science Foundation and other institutions sponsoring the work, are based on telescope observations and new computations of masses and velocities within the galaxies.

Alan Dressler of the Carnegie Institution and Douglas Richstone of the University of Michigan said the Andromeda galaxy, known as M31, has dark matter at its core that is about 70 million times the mass of the Earth's sun.

And M32, a satellite galaxy one-tenth the size of Andromeda that revolves around it, has about one-tenth as much dark matter concentrated at its center, they said.

The astronomers said huge black holes are the only known objects that could fit their calculations for this nonluminous matter at the heart of galaxies.

Black holes are concentrations of matter so dense that they have collapsed upon themselves and nothing, not even light, can escape their intense gravitational pull.

Scientists previously suspected that black holes or some other dense bodies are responsible for the great amounts of energy detected at the centers of the great star clusters known as galaxies. These suspicions, however, have been based upon circumstantial evidence, Dressler said.

"We have taken a big step from what was only a possible explanation to what is the most likely explanation," said Dressler, a staff astronomer at the Carnegie Institution's Mount Wilson and Las Campanas observatories.

Dressler used the 200-inch Hale telescope at the Palomar Observatory in California to measure the motion of stars near the centers of M31 and M32. By studying shifts in the frequency of their electromagnetic spectra, the astronomers calculated the velocities of the stars and distances from the galactic centers to find their masses.

Richstone, chairman of the astronomy department at Michigan, used advanced mathematical techniques to explain the orbital patterns seen for the stars in the galaxies.

The visible mass seen in the galaxies could not be responsible for the stellar dynamics the scientists observed, Richstone concluded.

M31 and M32 are 2 million light-years away from Earth, and Andromeda is visible to the naked eye as a faint glow high in the summer sky.