Space

Compact objects

In the chapter on star evolution we said that both in white dwarfs as well as neutron stars the gravitational collapse is opposed by a pressure which no longer depends on gas temperature, but on its density. In astrophysics bodies of this kind are called compact objects and the matter of which they are made of is called degenerate matter. In order to explain this behaviour we must go from the extremely huge field to the infinitely small one: here is where modern quantum mechanics come to help us. However one must not be surprised by the leap from one scientific discipline to another. The different scientific branches support each other and more and more often their discoveries overlap to give a single more complete picture of the situation. So, quantum mechanics say that atomic particles, such as protons, neutrons and electrons, have a series of proprieties which characterize them: mass, electric charge and certain numbers, called quantum numbers, that describe its energy. This kind of particles obey to a law called Pauli’s exclusion principle, from the name of the Austrian physicist who discovered it: two energetically identical particles cannot coexist in very little space.
In a white dwarf, matter is ionized and electrons are free of their atomic orbits around the nuclei.
During gravitational collapse, matter's density increases and so does the electron concentration within a certain space volume. Compression continues  until the electrons are able to assume energetic configurations that distinguish them. When all possible combinations have run out, according to Pauli's exclusion principle no other electrons can enter that given volume; thus a barrier is formed which prevents matter from collapsing any further. Therefore the white dwarf reaches its final stable configuration.
The same mechanism takes place also in neutron stars; during the final collapse the star’s elevated mass produces density so that atomic electrons and protons bind together to form neutrons. Thus the star is composed by a compact mass of heavy electrically neutral particles that is imploding under the gravity effect. Once again the quantum proprieties of these particles which, like electrons, are ruled by Pauli's exclusion principle. Therefore, once a certain threshold has been passed, neutron density stops the gravitational collapse, giving the star permanent stability.