During their lifetime, stars go through several phases that are correlated to their internal dynamics. This evolution can lead them to the white dwarf stage and, subsequently, to the neutron star and black hole stages. In 1930, when he was only 20 years old, the Indian physicist Subrahmanyan Chandrasekhar demonstrated that the end of stars depended on their mass and that of their core.
It then calculates the maximum limit that an object can reach before either exploding, in the case of a white dwarf (low mass stars), or collapsing into a neutron star or a black hole in the case of a massive star. This limit is called "Chandrasekhar limit and is 1.44 solar masses, or 2.9×10 30 kg.
The white dwarf stage is the final stage in the evolution of Main Sequence stars with masses between 0.05 and 10 solar masses (1). A star remains stable when the effect of gravitation, which tends to contract the star on itself, is counterbalanced by the radiation pressure resulting from the thermonuclear fusion reactions taking place in its core. These two forces balance each other, allowing the star to remain stable on the Main Sequence.
However, at the end of its life, a low-mass star has fused almost all of its hydrogen into helium. Thermonuclear fusion reactions therefore gradually stop and no longer provide enough radiation pressure to balance the gravitational contraction. The star then collapses on itself.
This collapse leads to an extremely large rise in core temperature (up to 100 million Kelvin) and pressure, leading to a new state of hydrostatic equilibrium and the ignition of new thermonuclear reactions fusing helium into carbon and oxygen, through the triple-alpha reaction mechanism (the process leading to the formation of carbon from the fusion of three alpha particles).
These fusion reactions release a large amount of energy from the center to the periphery of the star, generating an internal pressure responsible for increasing the diameter of the star:it becomes a red giant. This stage is temporary, as helium fusion is a fairly rapid process; once the latter is over, the gravitational contraction begins again. Since the star was not massive enough to initiate carbon fusion, its core collapsed and gave birth to a white dwarf. The peripheral layers bounce off the core and are ejected into space to form a planetary nebula (composed of hydrogen and helium).
The carbon-oxygen core of the white dwarf continues to contract under the effect of gravity. However, under this progressive compression, the atoms end up being very tightly juxtaposed. Matter reaches such a density that the atomic orbitals are compressed against each other. From then on, a pressure opposite to gravity appears:the electronic degeneracy pressure.
In effect, the Pauli exclusion principle then prevents the atomic orbitals containing electrons from coming any closer. This principle forbids two electrons to be in the same quantum state, i.e. to have an identical spin on the same energy level. To prevent the violation of the exclusion principle, a force opposing the gravitational contraction emerges. Such a state of matter is called "degenerate matter".
Thanks to this electronic degeneracy pressure opposing gravity, the white dwarf, whose core has a mass of less than 1 solar mass (2), reaches a state of very stable equilibrium. Without outside influence, the white dwarf is doomed to gradually cool down and become a black dwarf.
However, if the white dwarf belongs to a binary system, it can accrete (absorb) matter from its companion (3), having the effect of increasing its mass while decreasing its radius. This increase in mass leads to the compression of the core, leading to an increase in temperature until the latter is sufficient to allow the fusion of carbon (3).
When the mass of the white dwarf reaches the Chandrasekhar limit, i.e. 1.44 solar masses, the temperature is so high that the carbon fusion reaction suddenly goes into overdrive, releasing a phenomenal amount of energy (greater than the binding energy gravity of the core) and leading to the thermonuclear explosion of the white dwarf in type Ia supernova (4). The white dwarf is literally blown away and no residue, apart from a remnant, remains after its end. Such a process is called the "simple degeneracy model".
It has been argued that, theoretically, if a white dwarf accretes a large amount of matter in an extremely short period of time, then it could contain the carbon fusion reaction long enough to collapse in on itself and form a neutron star (5). However, this hypothesis is currently highly debated.
For a star with a mass greater than 10 solar masses, the fate is quite different. Such a star is massive enough for its carbon-oxygen core to contract and heat up to initiate the fusion of carbon into neon and magnesium. Then, as the temperature continues to rise under gravitational contraction, the neon fuses to form iron and nickel-56.
At this point, the pressure of electron degeneration within the heart is sufficient to counteract gravity. However, the fusion reactions continue to produce iron and nickel-56, which gradually deposit on the core, gradually increasing its mass until it reaches the Chandrasekhar limit. From then on, the pressure of electronic degeneration is no longer sufficient and the heart collapses.
In order to respect the Pauli exclusion principle, the electrons enter the protons which are transformed into neutrons. The core therefore undergoes a general neutronization with massive emission of electronic neutrinos. The core becomes a neutron star with a diameter between 20 and 30 kilometers.
At the same time, the layers surrounding the heart bounce off it at 10% the speed of light, forming a shock wave. The massive emission of neutrinos propagates from the center to the periphery, brutally heating the shock wave. The shock wave and the rapid neutrino emission combined lead to a phenomenal release of energy in the form of a type II supernova.
At this stage, the neutron core continues to contract under the effect of gravitation until the neutrons, subject to the Pauli exclusion principle, develop a repulsive force counteracting gravity:this is the degeneracy pressure. neutronics. The neutron star thus becomes stable as long as the core does not exceed a mass of 3 solar masses.
As seen above, a neutron star is stable as long as the neutron decay pressure counteracts the gravitational contraction. This is only possible as long as the mass of the core remains less than or equal to 3 solar masses. Beyond this limit calculated by physicists J. R. Oppenheimer and G. M. Volkoff, the neutron star collapses into a black hole.
For a solitary neutron star, the evolution will therefore be extremely stable. On the other hand, for a binary neutron star and/or surrounded by other celestial bodies, the evolution is more chaotic. This will be able to accrete matter from its companion(s), gradually increasing the mass of its neutron core to the Oppenheimer-Volkoff limit.
Once this limit is reached, the neutron degeneracy pressure is no longer sufficient to counterbalance the gravitational contraction. The core then collapses on itself and an event horizon appears, trapping the light emitted during the release of energy due to the collapse. The neutron star disappears to make way for a stellar black hole.
In some cases, the transformation into a black hole does not go through the neutron star stage. If during neutronization the core of the star has a mass greater than the Oppenheimer-Volkoff limit, then the core collapses directly into a stellar black hole.
