ASTRONOMERS pursuing one of the most elusive cosmic objects - a black hole - have had good hunting. They reported three likely candidates already this year.
Moreover, they found two of them with the Hubble Space Telescope. This shows that the telescope still can carry out one of its primary search missions in spite of its flawed mirror.
A black hole is an object that has collapsed to such a high density that nothing - not even light - can escape its strong gravity. It has literally disappeared from sight. Astronomers can only infer its possible presence from the effect of its gravity on nearby visible objects.
That's how Tod Lauer and C. Roger Lynds of the National Optical Astronomy Observatories in Tucson, Ariz., and Sandra Faber of the University of California at Santa Cruz located what they think may be a black hole with 2.6 billion times the mass of our sun at the core of a giant galaxy known as M87.
They announced the discovery at an American Astronomical Society meeting in Atlanta in January.
In February, Philip Charles, J. Casares, and T. Naylor, working at the Royal Greenwich Observatory on Tenerife in Spain's Canary Islands, described in the magazine Nature what they consider "the most persuasive case yet for the existence of a black hole" in the constellation Cygnus. It is a stellar weight object with about 6.26 solar masses.
Then, on April 8, the Space Telescope Science Institute in Baltimore reported that Drs. Faber and Lauer had located a second possible galactic black hole with 3 million solar masses at the center of galaxy M32. This is a small companion galaxy to the famed giant Andromeda galaxy, which dominates the galaxy cluster to which our own Milky Way galaxy belongs.
Astrophysicists are eager to find black holes for their own sake and as objects which permit the study of physics under extremely strong gravitational conditions. Space telescope researcher Joseph Dolan of the Goddard Space Flight Center in Greenbelt, Md., explains that physicists have to use both Einstein's theory of general relativity and quantum mechanics to describe black holes. They have not been able to link these two theories together. Dr. Dolan says that, if astronomers can reliably identify a bl ack hole, it should provide a "useful test bed on which to verify and refine attempts to unify the two theories."
Dolan explains that it is a question of the "level of proof." The new discoveries have produced some of the most telling evidence of black holes yet found. However, Dolan says, "We have not yet reached a level where we can say with reasonable certainty that a black hole exists." Lauer emphasizes the same point. Referring to the great mass at the center of M87, he said: "It looks like a duck, but we haven't heard it quack yet."
Faber, Lauer, and their colleagues infer the presence of black holes in M32 and M87 by noting the massive concentration of stars toward their centers. These concentrations appear as small bright regions in space telescope images. This strongly suggests the influence of very massive central objects.
Dr. Charles and his colleagues are studying an object, called V404 Cygni, that is part of the binary star system. They infer a black hole's presence from the system's movements and X-ray emissions.