Astrophysical Black Holes in the Physical Universe
In modern astronomy, the mystery of black holes (BHs) attracts extraordinary interest for both researchers and the general public. Through the 1930s, the applications of general relativity and quantum mechanics to the studies of the late evolution of stars predicted that stars with different initial masses, after exhausting their thermal nuclear energy sources, may eventually collapse to become exotic compact objects, such as white dwarfs, neutron stars, and BHs. A low-mass star, such as our Sun, will end up as a white dwarf, in which the degeneracy pressure of the electron gas balances the gravity of the object. For a more massive star, the formed compact object can be more massive than around 1.4 solar masses (M⊙), the so-called Chandrasekhar limit, in which the degeneracy pressure of the electron gas cannot resist the gravity, as pointed out by Chandrasekhar. In this case, the compact object has to further contract to become a neutron star, in which most of the free electrons are pushed into protons to form neutrons and the degeneracy pressure of neutrons balances the gravity of the object, as suggested by Zwicky and Landau. Then as Oppenheimer and others noted, if the neutron star is too massive, for example, more than around 3 M⊙, the internal pressure in the object also cannot resist the gravity and the object must undergo catastrophic collapse and form a BH.