FOR several decades, astronomers have looked for something they know that, in principle, they cannot see - a black hole. Now a Hubble Space Telescope research team has reported the next best thing to absolute confirmation that such a thing does exist. This is a triumph for the art of doing astronomy by indirection.
A black hole is a highly condensed object whose gravity is too strong for anything - including light - to escape from its immediate vicinity. Many astronomers think such an object is the power house at the core of so-called active galaxies. Black holes pour out massive energy in radiation and sometimes jets of matter.
How can this be? If nothing escapes a black hole, how can it be an energy source? Actually, the energy doesn't come from the black hole itself. It comes from matter falling into it. Stars, dust, and gas caught in the hole's gravitational grip orbit around it and spiral down into it. Collisions and friction heat this fast-moving material. Before it disappears into the hole forever, this material gives up vast amounts of gravitational energy. Just as water flowing down a hydroelectric dam gives up gravitational energy to make electricity, the infalling matter can provide the energy to power active galaxies.
Thus astronomers can infer a black hole's presence indirectly from its powerful effects on its surroundings. In an active galaxy, they look for clustering of stars that may be orbiting a black hole. They also look for what they call an accretion disk of fast rotating dust and gas caught in the hole's gravity. Most important, they want to measure the speed with which such orbiting matter is moving. Then, using Isaac Newton's law of gravity, they could calculate the central mass that has captured the orbiting material. In effect, they could ``weigh'' the source of the powerful gravity. And if it is substantially heavier than any visible matter at the galaxy center, that would be strong evidence for a black hole.
In recent years, astronomers have begun to find what they are looking for. In Nov. 1992, for example, Holland Ford of The Johns Hopkins University and the Space Telescope Science Institute in Baltimore and colleagues released what they called the ``best view yet'' of an accretion disk. It is in a galaxy called NGC 4261. They thought it probably orbits a central black hole with a mass 10 million times that of our sun. But, since they couldn't measure the rotational speed of the disk, they couldn't actually weigh that mass.
Two weeks ago, Dr. Ford and colleagues presented an image of the galaxy M87 that included that crucial kind of data. The sharper sight of the repaired Hubble telescope not only clearly imaged an accretion disk, it allowed direct measurement of the speed at which the orbiting gas is traveling. It whizzes along at 1.2 million miles an hour (550 kilometers a second).
Only a mass of some three billion suns concentrated in a volume no larger than our solar system could hold this fast-moving material in orbit. There's no visible indication of such a central mass. ``If it isn't a black hole, then I don't know what it is,'' Ford says. The long search by indirection has finally paid off.