If the terminal stages of star death leave a remnant star mass greater than 3 solar masses, the ultimate gravitational collapse will produce a "black hole", a relativistic singularity. A black hole is a localized region of space from which neither matter nor radiation can escape. The Uhuru Satellite (uhuru is the Swahili word for "freedom"), launched in 1970 and operating until 1973, was the first artificial satellite for x-ray astronomy, producing a catalogue containing 339 x-ray sources. ... ... G. Bisnovatyi- Kogan (Space Research Institute Moscow, RU) reviews the papers presented at a recent international conference on black holes (11-17 January 1998, S.N. Bose National Centre for Basic Sciences Calcutta, IN). This was perhaps the first conference devoted to black holes as real astronomical objects. The first indications for the actual existence of black holes came from x-ray observ-ations of the UHURU satellite, but black holes were predicted theoretically 60 years ago, and in fact Laplace in 1796 already noted that light cannot leave a star when its free-fall velocity as determined by the star's mass and radius is larger than the speed of light as we have measured it. At the present time, if the apparent mass of a discovered compact object exceeds the limiting mass, and if we hold the theory of general relativity to be valid, we say we have discovered a black hole. There are now 40 such objects that have been identified, 30 of them super- massive objects each containing millions and billions of solar masses and surrounded by dense stellar clusters in the strong gravity of a black hole.

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