A black hole is an object in space that has a gravitational pull that is so strong that light cannot even escape from its tremendous pull (NASA). Black holes are in invisible because light cannot escape from its clutches (World book at NASA). A black hole can form when a massive star runs out of nuclear fuel and is crushed by its own gravitational force. No one has ever found a black hole, so their existence is still uncertain and is based on inferences. Scientists assume that an object is a black hole when the invisible object emits x-rays and when an object that is around it orbits it with a very high velocity. Based on inferences, scientists have concluded that there are two types of black holes: galactic and supermassive. Galactic black holes are found in binary systems, in which a pair of stars orbit each other. Supermassive black holes are believed by scientists to be in the center of most galaxies. The surface of a black hole is called the event horizon and here the gravitational pull becomes extremely strong and nothing that gets sucked in ever comes out of it.

Important Early Contributors to the Theory of Black Holes

The modern day theory of black holes was formulated from Einstein's theory of relativity, which was published in 1916. so, you could basically say that
Einstein was one of the first people who contributed to the idea of black holes; however, Einstein was not the first person to come up with the idea of black holes. Einstein published two theories, both related to one another and these theories set the foundation for what is now known as the theory of black holes. Einstein's equations gave black holes their fundamental descriptions. Basically, the idea of black holes is based on complex equations and inferences. In 1968, John Wheeler, an American scientist coined the term "black hole"(Event Horizon). He named these mysterious objects this because of their mysteriousness and darkness. Reverend John Mitchell and Pierre-Simon Laplace were the first people to come up with the idea of black holes. In 1783, Reverend John Mitchell came up with gravity could affect light as well as matter. In 1795, Pierre-Simon Laplace reached the same conclusion as Mitchell, even though they did not work with each other. Even though they worked independently and did not receive help from each other they both believed in escape velocity. Escape velocity is the speed necessary to escape a star's gravity. Both of these scientists based their theories on Newton's theory of gravitation and corpuscular light. However their separate theories were both discarded by many scientists of their day. Then in 1916, Albert Einstein published his General Theory of Relativity, which finished his 1905 paper on the Speed Theory of Relativity. This theory is a very mathematically complex, but from these equations we can predict the existence of black holes. In 1916, Karl Schwarzschild introduced the modern concept of black hole. He did this by discovering a mathematical solution to Einstein's equations relating to the gravitational field of a point mass. This describes the space and time around any circle-shaped mass and it became known as Schwarzschild Geometry. This includes the distance from the center of sphere at which light cannot escape. So, basically if an object is entirely inside this distance (known as Schwarzschild Radius) we have a black hole. Those are some of the people who have contributed to the theory of black holes.

The Future Study of Black Holes

Since Schwarzschild's findings, the study of black holes declined until the 1960's. The reason why the 1960's renewed an interest in the study of black
holes is because of the fact that scientists began to send the first rockets up with X-Ray detectors on them. This helped scientists learn more about black holes and informed them about how many black holes there are in our galaxy alone. There was a satellite launched in the 1970's, called the Uhuru was sent up to study the entire sky in only X-Ray vision. This helped scientists realize that black holes are not the only objects in the sky that emit X-Rays. So, scientists used the gravitational pull of the objects and on the objects to determine if there is a black hole present. Since then there have been many more satellites sent up to study the night sky and what it contains. Scientists have discovered that black holes do not emit any detectable radiation directly, so they have to study the objects around them to detect them. Scientists have found a supermassive black hole in the center of our galaxy and they speculate that it is a 3 million-solar-mass object. Scientists have decided that black holes are apart of our galaxy and that will not change. Scientists have discovered that in our galaxy there are many black holes in binary systems and not all of them are supermassive like the ones usually found in the middle of galaxies.
In conclusion, scientists have learned a lot since the theory of black holes was formulated and they will continue to strive to learn more.