Black Hole is I think the most popular mystery in Space Science. The word "black hole" certainly thrills most of us. What is black hole ? Black Hole is a huge mass centered in a very small volume and definitely with very very high density. Generally dead star is the source of the mass forming black hole. Some scientists say that if a star undergoes "Supernova" state and explodes, the scattered mass collapses all together and starts forming black hole. With the time these mass become so dense that it occupies almost zero volume and infinite density. This state is known as absolute "singularity". Thus the gravitational filed of this cluster becomes so strong that nothing can come out of it. Even the light cannot escape. As we cannot locate this with emitted or reflected light, it seems dark and we mention this as black hole.
As there is virtually nothing that can come out of the black hole, we cannot see them normally. But these can be detected through some technical process. As the gravitational attraction of black holes is huge. Sometimes it happens that some starts are orbiting a region in space that looks absolutely empty. In these cases certainly there must be a doubt about the movement of the stars. There must be something that we cannot see normally. There must be a black hole and the existence of the black hole is affecting the orbit of the stars.
The black holes are totally blank space in case of light emission. But this neither says that there is no light emitting from the black hole nor the black holes are cold ones. As the clusters from space is continuously falling to the black holes, immense heat is being generated and so as drastic radiation. A huge emission of X-Ray can be observed with modern astronomical telescopes. These radiation emits along the axis of the black holes.
As large quantity of gas, particles and matters from nearby stars are continuously falling into the black hole. They form an exceptionally glowing light around the black hole. Center of which contains a black dark spot. These pictures has been taken from our satellites orbiting earth in outer space. The glow, thus created, is very flickering. Even this sources flickers hundred time in a second. And this flicker distinguish the glow around the black holes with the light source from various stars.
In 1783, John Michell first mentioned about black hole. The idea of black hole was supported by Pierre Simon Laplace in 1796 with black stars. Karl Schwarzschild first proved that black hole can theoretically exist. In this regards we must mention the general theory of relativity by Albert Einstein. There are many other scientists worked in this field and still today scientists are working with ultra modern equipments. There are many theories and counter theories exists regarding black holes.
The most exhilarating phenomenon about the black hole is Event Horizon. This is theoretical but still amazing. According to the general theory of relativity, event horizon is the boundary of space-time. This is such an area around black hole, within which no event can affect any external matter i.e. not residing within event horizon. Any matter or energy can never reach the observer. If any object travels towards a black hole, it slows down. As it approaches closer to the black hole, it travels with lesser speed. During this journey of the object, the light emitted from the object becomes more and more dim and red. Theoretically the object can never reach the surface of the black hole. Actually it will take infinite time to reach. A clock will tick slower near the black hole or rather near the event horizon.
There are a few differences in various black holes. They are categorized according to mass, movement and electrical charge contained within them. Supermassive, Intermediate-Mass, Stellar-Mass, Micro - are the segregation according to the mass. Rotational, Non-Rotational, Charged, Neutral - are general category according to movement and electrical charge.
You might be afraid, if someday the Sun becomes a black hole, it might swallow our earth. Please do not worry, if it happens the radius of Sun will be so small, the orbit of earth will remain unchanged and not at all going to crash into the sun. But there will be so many difficulties, we can hardly survive if we loose our beloved Sun.