What happens with neutron stars

Or that would be its fate, if not for neutron stars. If Zwicky was right that neutron stars can exist, then they must be rather analogous to white-dwarf stars, but with their internal pressure produced by neutrons instead of electrons. This means that there must be a neutronstar curve in Figure 5.3, analogous to the white-dwarf curve, but at circumferences (marked on the horizontal axis) of roughly a hundred kilometers, instead of tens of thousands of kilometers. On this neutronstar curve neutron pressure would balance gravity perfectly, so neutron stars could reside there forever.

Suppose that the neutronstar curve extends upward in Figure 5.3 to large masses; that is, suppose it has the shape labeled B in the figure. Then Sirius, when it dies, cannot create a black hole. Rather, Sirius will shrink until it hits the neutronstar curve, and then it can shrink no more. If it tries to shrink farther (that is, move to the left of the neutronstar curve into the shaded region), the neutrons inside it will protest against being squeezed; they will produce a large pressure (partly due to degeneracy, that is, “claustrophobia,” and partly due to the nuclear force); and the pressure will be large enough to overwhelm gravity and drive the star back outward. If the star tries to reexpand into the white region, the neutrons’ pressure will decline enough for gravity to take over and squeeze it back inward. Thus, Sirius will have no choice but to settle down onto the neutronstar curve and remain there forever, gradually cooling and becoming a solid, cold, black neutron star.

Suppose, instead, that the neutronstar curve does not extend upward in Figure 5.3 to large masses, but bends over in the manner of the hypothetical curve marked A. This will mean that there is a maximum mass that any neutron star can have, analogous to the Chandrasekhar limit of 1.4 Suns for white dwarfs. As for white dwarfs, so also for neutron stars, the existence of a maximum mass would herald a momentous fact: In a star more massive than the maximum, gravity will completely overwhelm the neutron pressure. Therefore, when so massive a star dies, it must either eject enough mass to bring it below the maximum, or else it will shrink inexorably, under gravity’s pull, right past the neutronstar curve, and then—if there are no other possible stellar graveyards, nothing but white dwarfs, neutron stars, and black holes - it will continue shrinking until it forms a black hole.