A NASA rendition of a Supermassive Black Hole
A NASA rendition of a Supermassive Black Hole

What is a Black Hole?
A black hole is a region where spacetime is so curved that anything that falls into it can never escape. The gravitational pull of a black hole is so powerful its' escape velocity exceeds the speed of light, which is the fastest moving particle in the universe. They are invisible because light is unable to get escape through the event horizon, which is the point between the inside of a black hole and the outside universe. A point would form in the center where spacetime is curved infinitely, which is referred to as a gravitational singularity. Astronomers acknowledge black holes are at the center of every galaxy, including our own Milky Way Galaxy, which they have named the black hole Sagittarius A*.(1) In total there are about 300 black holes in the Milky Way. (2) The number is increasing as scientists detect more every year. Black holes are classified by their mass, which is measured in solar masses.

There are four different types of black holes:
  1. A Supermassive black hole is the largest kind of black hole in the universe and have masses that range from thousands to billions of solar masses. Their size can be anywhere from about 150,000 km to 10 AU (about 1.5 billion km). Sagittarius A* is a supermassive black hole with a mass of about 410,000 stellar masses and about 14 million miles in length. A supermassive black hole in the heart of nearby galaxy 1H0707-495 spins so fast it absorbs about two earths every hour. (3)
  2. An intermediate-mass black hole is a type of black hole whose legitimacy has been highly debated among astronomers because they are extremely rare. The mass for one is between a few hundred and a few thousand stellar masses. They have a distance varying between a few hundred and 6,384 km, the equivalent of one Earth radius.
  3. The most common type of black hole is a stellar black hole, which usually have a mass between 2 and 15 stellar masses.
    (4) They have distances between 15 and 40 km across and were the primary source of research for Stephen W. Hawking, a very renowned cosmologist.
  4. Tiny black holes are the smallest types of black holes but their existence is called into question because of how small they are thought to be. They are so small that Hawking discovered these black holes could evaporate due to thermal radiation because they are only about 0.1 mm in size. (5)
A NASA Chandra photo of a black hole emitting x-ray jets 2/2/09
A NASA Chandra photo of a black hole emitting x-ray jets 2/2/09

History of Black Hole Research
The concept of black holes were first suggested to exist in 1783 when John Michell and Henry Cavendish of Great Britain and believed an object could be so compact that its escape velocity could exceed the speed of light. The first real attempt to really prove the existence of black holes was made by Albert Einstein with his theory of general relativity in 1916. The theory proves gravity has an effect on a wave of light, which was revolutionary at the time. Karl Schwarzschild developed the Schwarzschild radius which is the distance surrounding the event horizon in a black hole. This also led to the first belief that time could be slowed inside a black hole. (6) In 1930 Subrahmanyan Chandrasekhar calculated the maximum mass a white dwarf star could be, and sets the minimum mass requirement for a star to collapse into a black hole. The number is known as the Chandrasekhar Limit, and is equivalent to 1.44 solar masses.(7) In 1937 the Tolman–Oppenheimer–Volkoff limit was developed to describe how neutron stars with a mass over three solar masses would become black holes, and no laws of physics could stop an object from collapsing into a black hole. The term black hole was first coined in 1964 by Anne Ewing but didn't gain attention until John Wheeler used it on a regular basis. Wheeler developed the no-hair theorem which states that a black hole is described by three properties of angular momentum, electric charge, and mass. Stephen Hawking along with a few other scientists supplied mathematical statistics to confirm the no-hair theorem. In 1974 Hawking made a groundbreaking discovery when he discovered black holes emit thermal energy called Hawking radiation, because it was not known that black holes emitted anything.

How are Black Holes Created?
Black Holes form when huge stars with a mass over 25 times higher than the sun end their lives as a supernova, through one of the most powerful events in the entire universe. The direct cause of a black hole occurs after a supernova, when a star loses the fight against gravity. Gravity compresses the star's core into a point with basically zero mass; and also has infinite density which exemplifies gravitational singularity.(3) Milliseconds after the core collapses the newborn black hole immediately starts to swallow the star, the star then explodes and emits Gamma-ray bursts. They scatter through the universe at the speed of light and are used to count how many black holes are being created. Once the gamma-ray bursts have been emitted the black hole begins to continually grow in size by swallowing interstellar dust, stars and anything else. Black holes also emit thermal radiation called Hawking radiation which can inhibit its growth, and slowly cause it to evaporate if matter is not being absorbed.

A Hubble photo of a supernova, eventually it will become a black hole
A Hubble photo of a supernova, eventually it will become a black hole

2.Star Clusters Around Recoiled Black Holes in the Milky Way Halo.
Ryan M. O'Leary, Abraham Loeb. Sep 2008.
e-Print: arXiv:0809.4262 [astro-ph]
Chandrasekhar, S. (1998) [1983]. The Mathematical Theory of Black Holes. New York: Oxford University Press.