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A region of space where the gravitational force is so strong that not even light can escape
from it. Black holes are formed when matter collapses in on itself catastrophically so that
more than a critical quantity of mass is concentrated into a particularly small region.
Theory suggests that "mini" primordial black holes might have formed from large density
fluctuations in the conditions prevailing in the early universe. It is believed that stellar black holes may form when massive stars explode, if the central relic is more than three solar masses or exceeds that mass when material cascades back on to it. To create a black hole, several solar masses of material would have to be packed into a diameter of just a few kilometres. Matter falling into supermassive black holes is a favoured explanation for the exceptionally high energy production in certain active galactic nuclei and quasars. Direct observations of compact nuclei in galaxies, and measurements of the velocities of gas and stars near the centres of galaxies' lends weight to the idea that massive black holes do indeed exist at the centres of many galaxies. Black holes can never be observed directly: their existence can only be inferred from their gravitational effects and the radiation emitted by material falling into them. A number of stellar X-ray sources, such as Cygnus X-1, are binary star systems in which one component appears, from determinations of its mass and luminosity, to be a black hole. In such systems, observations of the visible star make it possible to compute the orbit and mass of its dark, compact companion. The X-rays result from energy released as matter streams on to the compact star. See also: Kerr metric, Schwarzschild radius. |