How small a neutron core can be

“How small can the mass of a neutron core be?” Oppenheimer and Serber were thus driven to ask themselves. “What is the minimum possible mass for a neutron core?” Notice that this is the opposite question to the one that is crucial for the existence of black holes; to learn whether black holes can form, one needs to know the maximum possible mass for a neutron star.

In his article Landau, also aware of the importance of the minimum neutron core mass, had used the laws of physics to estimate it. With care Oppenheimer and Serber scrutinized Landau’s estimate. Yes, they found, Landau had properly taken account of the attractive forces of gravity inside and near the core. And yes, he had properly taken account of the degeneracy pressure of the core’s neutrons. 

But no, he had not taken proper account of the nuclear force that neutrons exert on each other. That force was not yet fully understood. However, enough was understood for Oppenheimer and Serber to conclude that probably, not absolutely definitely, but probably, no neutron core can ever be lighter than of a solar mass. If nature ever succeeded in creating a neutron core lighter than this, its gravity would be too weak to hold it together; its pressure would make it explode.

A neutron core is nothing but a neutron star that happens, somehow, to find itself inside a normal star.

What, precisely, is the fate of massive stars when they exhaust the nuclear fuel that, according to Bethe and Critchfield, keeps them hot? Which corpses will they create: white dwarfs? neutron stars? black holes? others?

Chandrasekhar’s calculations had shown unequivocally that stars less massive than 1.4 Suns must become white dwarfs. Zwicky was speculating loudly that at least some stars more massive than 1.4 Suns will implode to form neutron stars, and in the process generate supernovae.