Lecture 21 - Maximum mass of neutron stars

Academic pursuit has always been influenced by the course of human society and never more so than the effect of the two world wars on the twentieth century physics. Before the world came to know Robert J. Oppenheimer as the Gita-quoting head of the 'Manhattan Project', his attention was focussed on something entirely other-worldly. Oppenheimer, along with his students Volkoff and Snyder, was busy calculating the maximum mass of neutron stars.

Today, advances in nuclear physics has but only validated and refined their pioneering work. Their calculations also led them to the conclusive prediction of Black Holes, but then the war began.

By 1934, Chandrasekhar had clearly established that white dwarfs cannot exist with mass greater than a limiting mass, now referred to as the Chandrasekhar Limit. Soon after the ‘neutron’ was discovered, physicists made stars entirely of neutrons, and it appeared for a while that very massive stars will find their ultimate peace as neutron stars. But this hope was dashed in 1938 when Oppenheimer and his student Volkoff proved that there is limiting mass for neutron stars also, analogous to the Chandrasekhar limit. This once again raised the question “What will be the fate of very massive stars?” In a historic paper published in 1939, Oppenheimer and Snyder proved emphatically that massive stars will end their lives as Black Holes, predicted by Einstein’s General Theory of Relativity. But Einstein summarily rejected the notion of black holes. This lecture is devoted to these dramatic developments.