In this second part of the article dedicated to black holes, I will describe how they are formed, as well as the different kinds of black holes the theory has been able to predict.
Formation of black holes
Two different processes can lead to the formation of a black hole: gravitational collapse, and high energy collisions.
The first process, gravitational collapse, occurs when the internal pressure of an object is insufficient to resist the object’s own gravity. Such a phenomenon is encountered when a cold star has a mass of more than about one and a half times that of the Sun; this limit was calculated by Subrahmanyan Chandrasesekhar and is known as the Chandrasekhar limit. Basically, when the temperature of a star can’t be maintained through stellar nucleosynthesis (in other words, the star ran out of fuel), the star starts to collapse under its own weight. Sometimes, the collapse might stop according to the Pauli exclusion principle and lead to the formation of a compact star. If the mass of the remnant star exceeds about 3 solar masses (this limit is known as the Tolman-Oppenheimer-Volkoff limit; there are uncertainties about its precise value because the physics of extremely dense matter are not well known), then nothing can stop the collapse. This will eventually lead to the formation of a stellar black hole. Once a black hole has formed, it will keep absorbing matter from its surroundings: this will lead to the formation of supermassive black holes, found in the centers of most galaxies.
The second process, high-energy collisions that create sufficient density could also create black holes. Recently, researchers hoped they would observe the formation in the Large Hadron Collider (these black holes would have evaporated almost instantly), in vain; this led many specialists to completely abandon the possibility that black holes could be formed in such processes.
Stephen Hawking showed, in 1974, that black holes are not completely black, and should emit particles in a perfect black body spectrum through thermal radiation: this effect is known as Hawking Radiation. If this theory is correct, black holes should then lose mass (mass being highly condensed energy according to the theory of relativity), then shrink and evaporate over time. The larger the black hole, the smaller the Hawking radiation, as the temperature of the spectrum is proportional to the surface gravity of the black hole: even stellar black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation; only extremely small black holes, a fraction of millimeter in diameter, would evaporate (such as the ones expected to form in the LHC during high-energy collisions).
Different types
There are different kinds of black holes, identified by their physical properties. The simplest kind of black hole is characterized by only one parameter, its mass. Such black holes have neither electric charge nor angular momentum: they are often referred to as Schwarzschild black holes, after Karl Schwarzschild who found this solution after calculating the gravitational field of a point mass in 1915.
More general solutions have been found: the Kerr black hole (named after Roy Kerr) describing black holes with angular momentum but no charge, the Reissner-Nordström black hole describing charged black holes with no angular momentum, and finally the Kerr-Newmann black hole with both angular momentum and electric charge.
Event horizon
The most important feature of a black hole is its event horizon: this is the boundary in spacetime from which, once passed (in the direction of the the black hole), nothing can ever come back, not even light (because of the escape velocity being greater than the speed of light).
In the case of the Schwarzschild black hole, the event horizon has a spherical shape. For rotating black holes, the event horizon is distorted and non-spherical.
In the third and last part of this article, I will describe what would happen to an observer looking at a black hole, as well as traveling through it (passing the event horizon).
