ASTRONOMERS pursuing one of the most elusive cosmic objects - a black hole - must make a long-postponed philosophical decision. After decades of inconclusive searching they now are amassing circumstantial evidence so fast they have to decide how strong that evidence must be to know, with reasonable certainty, that this unseen object actually exists.
A black hole is a mass that has collapsed to such high density that nothing - not even light - can escape its strong gravity.
Relativity theory predicts that any object with more than about three times the mass of the sun will reach this black-hole state when it collapses under its own gravity.
Since nothing can betray a black hole's existence, astronomers infer its possible presence by indirection. They look for gravitational effects on visible bodies and telltale radiation from matter falling into its maw.
Such matter can release vast energies. Black holes millions to billions of times the sun's mass may lie at the core of active galaxies and other cosmic centers of strong radiation. At the other extreme, a black hole of three to 10 solar masses could be the invisible member of systems in which a visible star orbits an unseen companion.
New research methods and more effective equipment, including the Hubble space telescope, have sharpened the search for black-hole candidates. Possible black holes with 3 million and 2.6 billion solar masses appear to lie at the cores of galaxies M32 and M87. A small black hole seems to be the invisible partner of V404 Cigni binary star system. There are at least five candidates. And as the space telescope eyes more galaxies, its detailed images reveal more evidence of central supermassive objects. In Ma y, the Hubble team reported finding one in the Whirlpool galaxy. Incredibly, a roughly shaped natural "X" marks the spot.
Galactic black holes betray their presence by radiation and gravitational effects. Star concentrations toward the centers of M32 and M87 suggest the influence of massive central objects. These are strong signs that they are indeed black holes.
The same is true of the Whirlpool galaxy's central object. Co-discoverer Holland C. Ford of Johns Hopkins University says one arm of the "X" probably is the rotating dust and gas ring that theory predicts will circle a black hole.
Evidence for small black holes in binary stars also is not conclusive. Among other things, astronomers want to know that the candidate object has more than three solar masses. This is hard to establish.
However, Ronald Remillard of the Massachusetts Institute of Technology, Jeffrey McClintock of Harvard University, and Charles Bailyn of Yale University now believe they can show that such an object in the binary system Nova Muscae 1991 is massive enough to qualify.
Astronomers also have identified a pattern of X-ray emission that seems unique to double-star black holes. This will enable them to find candidates quickly, giving them perhaps several hundred systems for study.
This could be the decade when one of the most exotic predictions of Einstein's relativity theory is transformed from a theoretical concept into accepted observational fact.