California astrophysicists have found powerful evidence that at the center of the Milky Way galaxy, deep inside a hollow cluster of stars, there lurks a gigantic black hole -- a super-dense object containing 4 million times the mass of the sun.

The nature of the galactic hub, around which great spiral arms wheel, has long been a mystery because its stars cluster so close together that it is hard to see into their center.

One prominent idea has been that the center is simply a region thick with stars. As the concept of the black hole developed, some scientists wondered whether a giant version of these curious objects might lie at the heart of galaxies. Black holes are large stars that have burned all their nuclear fuel and collapsed into a center so dense that its gravity prevents anything from escaping, even light.

The new findings, made by a team led by Nobel laureate Charles H. Townes at the University of California, Berkeley, are based on measurements of the behavior of interstellar gases deep inside the galaxy's central star cluster. The gas atoms, like all atoms, emit characteristic electromagnetic waves. Townes and his colleagues have found a way to use infrared radio telescopes to detect the waves radiating from these gases and to track their motion.

The Berkeley group could not detect the center but did find a ring of gases orbiting it. Gas atoms nearest the center were moving much faster than atoms farther away. The calculated differences, Townes reported in last week's Nature, the British science journal, would be most readily explained if at the center of the ring there were a source of gravity containing a mass equal to 4 million suns.