formed from a diluted but lumpy mixture of hydrogen and helium gas - the main components which formed the Big Bang. The galaxy in which we live, usually referred to as the Milky Way, is a giant spiral assembly of several billion stars, including the sun. The galaxy is now known to be an immense disk shaped object, with its visible disk having a diameter of about 1,000 light-years and a height above its principal plane of about 1,00 light-years. Most astronomers believe that galaxies like the Milky Way were formed from a large cloud of gas, which collapsed and broke up into individual stars. The stars are packed together most tightly in the center, or nucleus. Scientists believe it is possible that at the very center there was too much matter to form an ordinary star, or that the stars which did form were so close to each other that they coalesced to form a black hole. It is argued that really massive black holes, equivalent to a hundred million stars like the Sun, could exist at the center of some galaxiesPhotons always travel at the speed of light, but they lose energy when traveling out of a gravitational field and appear to be redder to an external observer. The stronger the gravitational field, the more energy the photons lose because of this gravitational red shift. The extreme case is a black hole where photons from within a certain radius lose all their energy and become invisible....
