The theory of black holes was conceived by Karl Schwarzschild during World War II.  The term black hole for the then-theoretical celestial object was coined later by John Wheeler.  Black holes are thought to have the escape velocity faster than the speed of light, which means not even light can escape their gravitational fields. Currently, most astronomical scientists have reached the consensus that black holes exist at the center of every galaxy.
Black holes can only be described by their spin, charge, and angular momentum, with other attributes derived from the basic properties. They are thought in classical cosmology, i.e. the Big Bang model, to be the result of collapsing matter following the explosion of large stars into supernovae. Therefore, the mass of a black hole is often depicted in terms of solar mass, denoted by m⊙.ּּ
With their basic physical properties, four types of black holes have been proposed by theoretical physicists, with each type named in honor of them: 
|Schwarzchild black hole||No||No|
|Kerr black hole||No||Yes|
|Kerr-Newman black hole||Yes||Yes|
|Reissner-Nordström black hole||Yes||No|
The Big Bang Interpretations
The idea of the universe starting out from an atom originated from the Belgian physicist contemporary to Einstein's time, George Lemaître. On March 28, 1949, the English astronomer Fred Hoyle popularized the phrase the "Big Bang" during a defense.  The framework of the Big Bang Theory and nuclear physics was later constructed into the cosmic interpretations of the theoretical celestial object.
Two types of black holes are categorized in the Bang Bang model according to the origins, primordial black hole and the normative black hole from stellar remnants. Primordial black holes are thought to be created not soon after the Big Bang, and the black holes from stellar remnants are thought to be created after a star exhausted its capacities for nuclear fusion. It is estimated that for a star to be capable of compaction into a singularity, it must have a mass greater than 3.4 times that of the Sun.
Quite a few features have been attributed to black holes in observational astronomy, among which include its Bolometric luminosity, denoted by LBol, absolute magnitude, the widely known event horizon and singularity, Hawking radiation and Hawking points, and active galactic nuclei, etc.  The stellar remnant belief of black hole postulates that the event horizon is the threshold in space where the gravitational force surpasses the velocity of light, and relativity theory postulates the singularity being a point of infinite spacetime curvature. To an outside observer, objects falling into a black hole will take an infinite amount of time to reach the event horizon. The amount of time as measured by the object falling into the black hole, however, can be very short.
Metrics in the Quantum Realm
Kerr's exact solutions of general relativity postulate that rotating black holes, namely Kerr and Kerr-Newman black holes, have two event horizons. Beyond the outer event horizon are the inner event horizon and ergoregions.  The ergoregions are composed of the outer ergosphere and inner ergosphere, beyond which are the ring singularity and singularity. 
The astronomical developments in black hole detection started the quantization. James M. Bardeen, Brandon D. Carter, and Stephen W. Hawking formulated four laws concerning black hole thermodynamics for the foundation of cryogenic technology applied in quantum sensing.  Even though Hawking proposed black hole evaporation theories before, contradictions emerged with the observational confirmation of Hawking's surface area law by Isi and his colleagues, after gravitational wave detection by Laser Interferometer Gravitational-Wave Observatory (LIGO) located in the U.S. became feasible. 
Observation of Black Holes
The concentration of mass by black holes with their basic properties has made gravitational lensing the optimal technique for black hole observation.  A multitude of spectra has been adopted in the surveys, and observation by gravitational wave detection with interferometry has been the recent development since 2015, apart from Virgo in Italy.  Like all other astronomical observations, the ideal site for detection is beyond the earth's atmosphere. The preferences of eliminating detection biases from the atmospheric environment have categorized telescopes into ground-based and space-based. Placing telescopes in the aerospace mounted on high altitude planes has also been adopted. 
While black hole mergers are easier to detect than isolated black holes, black holes with spins are easier to detect than those without, especially Schwarzchild black holes.  Black hole observations typically involve accretion disk, event horizon, long tail of stars, black hole shadows, black hole seeds, etc.   
The first black hole ever discovered was Cygnus X-1 in 1964. Cygnus X-1 is located within the Milky Way in the constellation of Cygnus, the Swan.  While early detections mainly utilized the X-ray and gamma-ray emitted from the believed accretion activities, infrared astronomy developed in 1967 when the Mauna Kea Observatories site for astronomy was founded 4200 m above sea-level on an extinct volcano in Hawaii. 
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