Black hole

A black hole is a region of space from which nothing, not even light, can escape. According to the general theory of relativity, it starts existing when spacetime gets curved by a huge mass. There is a sphere around the black hole. If something goes inside the sphere, it can not leave. This sphere is called the event horizon. A black hole is black because it absorbs all the light that hits it. It reflects nothing, just like a perfect black body in thermodynamics. Under quantum mechanics, black holes have a temperature and emit Hawking radiation, which makes them slowly get smaller.
Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. In 1916, Karl Schwarzschild found the first modern solution of general relativity that would characterize a black hole. David Finkelstein, in 1958, first published the interpretation of "black hole" as a region of space from which nothing can escape. Black holes were long considered a mathematical curiosity; it was not until the 1960s that theoretical work showed they were a generic prediction of general relativity. The discovery of neutron stars by astronomers in 1967 sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality. The first black hole known was Cygnus X-1, identified by several researchers independently in 1971.
Black holes of stellar mass form when massive stars collapse at the end of their life cycle. After a black hole has formed, it can grow by absorbing mass from its surroundings. Supermassive black holes of millions of solar masses may form by absorbing other stars and merging with other black holes. There is consensus that supermassive black holes exist in the centres of most galaxies, such as a supermassive black hole of more than four million solar masses is near the Sagittarius A* part of the Milky Way galaxy. 
There are three types of black holes: stellar black holes (between 1 and 100,000 solar mass), supermassive black holes (between 100,000 to 100 million solar mass) and ultramassive black holes (between 100 million to 100 billion solar mass).
== See also ==
* List of black holes
* Event horizon
* Stellar black hole
* Supermassive black hole
== External links ==
* Black Hole Comparison on YouTube
* https://www.space.com/15421-black-holes-facts-formation-discovery-sdcmp.html
Category:Astronomical Objects
Category:Astronomy
Category:Black holes
Category:General relativity