All the stars do not shine, the universe is home to many dark stars. Black holes are very massive objects whose gravitational field is so intense that it prevents any form of matter or radiation to escape.
Black holes are described by the theory of general relativity. When the heart of the dead star is too massive to become a neutron star, it shrinks inexorably to form this mysterious astronomical object, what the black hole. Stellar black holes have a mass of a few solar masses to billions of solar masses.
Black holes are born as a result of the gravitational collapse of massive stars residue. When the combustion by thermonuclear reactions in the heart of the massive star ends for lack of fuel, a supernova appears. It can leave behind a heart that continues to crumble quickly. A man already predicted in the 18th century, the existence of dark stars, physicist, astronomer and British geologist John Michell (1724-1793). In his notes he writes that when a star becomes too massive, it attracts the light under the influence of the gravitational force.
However, as his calculations give the star density corresponding to 18 billion tons in 1 cm3, he concludes that it can not exist. Today, black holes theory stipulates that it is so dense that their escape velocity is greater than the speed of light. Light cannot thus overcome the gravitational force of surface and remains trapped inside the black hole. The Milky Way is the central region of our Galaxy. It houses in its center, a black hole. This black hole of several million solar masses, is called Sagittarius A (Sgr A), it is located 27000 light years from us.