In 1916, the German astronomer Schwarzschild, with the support of Einstein's works, calculated the size of a star whose escape velocity would be equal to the speed of light .
This speed is an absolute limit, and according to relativity, nothing can go faster.
What's more, we know that the size of a neutron star decreases when its mass increases, because gravity becomes stronger than the degeneracy pressure.
If the mass of such a star increases, there comes a time, called the Oppenheimer-Volkoff limit, when the escape velocity becomes equal to the speed of light, and nothing will be able to escape the star.
Let us notice that the cohesion of a neutron star is also depending of the strong nuclear interaction. As the behaviour of this interaction is poorly understood under a high gravity, the Oppenheimer-Volkoff limit is not precisely known. It is contained between 1.5 and 3 solar masses.
On this graph, you can see that the size of a black hole horizon (the Schwarzschild radius) is equal to a neutron star size when its mass is about 2.5 Solar masses (this mass is called the Oppenheimer-Volkoff limit).
Notice that, for a white dwarf or a neutron star, the size decreases when the mass increases !
If nothing, not even light, can't escape, this star becomes invisible : Such an object is called a black hole.
The black hole has no material surface ; the original matter of the star is shrunk to an infinitely dense point, called a singularity.
The "surface" of the black hole is called the horizon, its size is called 'Schwarzschild radius'.
| If G is the gravitational constant and M the mass of a body whose radius is R, the escape velocity is  .
In the case of a black hole, this velocity is equal to c the speed of light,
hence the Schwarzschild radius of a black hole is expressed as  |
|
Everything which could happen beyond the horizon is trapped, and can only increase the mass of the black hole.
Contraty to Hollywood movies, a black hole is not a "cosmic vacuum cleaner" : it can only catch objects which come very near. If we replaced the Sun with a black hole, we couldn't notice the difference (at least in terms of gravity, we would miss the heat !)
We can view two scenarios to explain the creation of a black hole :
- a neutron star is part of a binary system, and its mass increases by the pulling of the matter of the other star - accretion disk phenomenon, like a nova.
- the collapse of a hypernova leads to the direct formation of a black hole.
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