Once Before Time - Martin Bojowald [95]
NEUTRON STARS: THE LAST UPRISING
Is a white dwarf finally the stable answer to gravity? As the upper limit of one and a half times the solar mass suggests, this is not the case. Nuclear reactions render Pauli’s exclusion principle inapplicable if the mass of a dying star is above that maximum for white dwarfs, named the Chandrasekhar limit after Subrahmanyan Chandrasekhar, who was awarded the 1983 Nobel Prize in Physics. When a large mass of matter is compressed in a star, electrons and protons of the elements come very near. (The exclusion principle does not hold for different types of particles.) If they are sufficiently close, a reaction occurs similar to that of two protons fusing. In a somewhat reversed order, this process is also observed in beta decay of radioactive nuclei, in which a neutron decays into a proton. In our case, however, an electron and a proton react and disappear, forming in their place a neutron together with a neutrino, which, because of its weak interaction with matter, can immediately leave the star. Neutron stars owe their existence to this ultimate uprising of matter against gravity.
Under normal circumstances, this reverse beta decay is in principle possible, but the neutron would decay back to a proton and an electron (plus antineutrino) just after a mere quarter of an hour. The neutron is heavier than the other two particles taken together and thus requires more energy to be stabilized. In a very dense star, gravitational collapse provides exactly the energy necessary to stabilize a neutron. And not just one neutron is stabilized—all protons and electrons in the star’s interior can easily combine to become neutrons. Again, one particle, the neutron, takes up less space than a proton and an electron together; thus, after the reaction, the star can collapse further and release energy.
This happens explosively. All of a sudden, the star’s central region collapses, with an emission of a huge amount of energy accompanied by a flash of the new neutrinos, which is difficult to detect but highly energetic. Large fractions of the star’s exterior regions, mere pawns in the violent game of inner processes, are tossed out into space when the sudden impact, brought about by the outer matter collapsing onto the newly formed hard neutron core, causes a shock wave traveling outward. Such explosions as the grandiose departures at the end of an active star’s lifetime are widely visible as supernovae. More precisely, these are supernovae of type II (or Ic if no hydrogen is left in the outer shells). They are to be distinguished from supernovae Ia, as encountered in the previous chapter, by virtue of their important role in modern cosmology; the latter are caused by a different kind of thermonuclear explosion of white dwarfs.
What is left in the core is matter even more uncommon than that in a white dwarf: almost pure neutrons. Here, no electric forces are acting, because all particles are neutral. The only stabilizing force is the quantum mechanical repulsion of neutrons, again based on Pauli’s principle, which does not care about the absent electric charge. Compared to a white dwarf, a neutron star is even denser, for from two charged particles, an electron and a proton, a single neutral neutron was formed. The wave functions of all neutrons