o there is someX-ray sources which have all the properties described above. Unfortunately itis impossible to distinguish between a black hole and a neutron star unless wecan prove that the mass of the unseen component is too great for a neutronstar. Strong evidence was found by Royal Greenwich Observatoryastronomers that one of these sources called Cyg X-1 (which means the firstX-ray source discovered in the constellation of Cygnus) does indeed contain ablack hole. It is possible there for a star to be swallowed by the black hole.The pull of gravity on such a star will be so strong as to break it up into itscomponent atoms, and throw them out at high speed in all directions.Astronomers have found a half-dozen or so binary star systems (two starsorbiting each other) where one of the stars is invisible, yet must be there sinceit pulls with enough gravitational force on the other visible star to make thatstar orbit around their common center of gravity and the mass of the invisiblestar is considerably greater than 3 to 5 solar masses. Therefore these invisiblestars are thought to be good candidate black holes. There is also evidence thatsuper-massive black holes (about 1 billion solar masses) exist at the centersof many galaxies and quasars. In this latter case other explanations of theoutput of energy by quasars are not as good as the explanation using asuper-massive black hole. A black hole is formed when a star of more than 5solar masses runs out of energy fuel, and the outer layers of gas is thrown outin a supernova explosion. The core of the star collapses to a super denseneutron star or a Black Hole where even the atomic nuclei are squeezedtogether. The energy density goes to infinity. For a Black Hole, the radiusbecomes smaller than the Schwarzschild radius, which defines the horizon ofthe Black Hole: The death explosion of a massive star, resulting in a sharpincrease in brightness followed by a gradual fading. At peak light output,...
